Fish care calculator

Aquarium Filter Flow Rate Calculator | FishZone

Never buy an underpowered filter again. Calculate exactly how much flow your aquarium needs to keep water clear, ammonia zero, and fish healthy — based on your specific tank size, stocking density, and fish species.

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  • Reviewed by Experts
  • Updated May 2026
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Beginner

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Quick Answer

Aquarium filtration is measured in Gallons Per Hour (GPH) or Litres Per Hour (LPH) of flow. The standard rule is that your filter must cycle your entire tank volume at least 4 to 6 times per hour for a typical community setup, and up to 10 times per hour for messy, heavily stocked, or predator tanks. A 50-gallon community tank needs a filter rated for at least 300 GPH — but because manufacturer flow ratings are tested with empty filters at zero head height, the real-world flow after adding media and accounting for vertical lift drops by 30–50%. Always buy a filter rated significantly higher than your calculated minimum. This calculator applies the correct multiplier for your stock level and outputs a safe recommended GPH or LPH target.

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How to Use This Calculator (Worked Example)

This tool requires 4 key inputs:

  1. Aquarium Volume: Enter your specific value (e.g., from your tank's test kit or dimensions).
  2. Unit: Enter your specific value (e.g., from your tank's test kit or dimensions).
  3. Stocking Density / Bioload: Enter your specific value (e.g., from your tank's test kit or dimensions).
  4. Planned Filter Type: Enter your specific value (e.g., from your tank's test kit or dimensions).

Example: If you input average baseline values, the calculator will immediately process the formula and return the recommended output and safety warnings above.

Method

This tool uses the visible inputs on the page and returns practical aquatics guidance for maintaining your tank.

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The Problem

Aquarium filter selection is consistently one of the most confusing and consequential decisions a fishkeeper makes. Manufacturer GPH ratings are misleading because they are measured without media and at zero head height, causing hobbyists to buy filters that deliver far less real-world performance than advertised. Under-filtered tanks accumulate ammonia invisibly until fish begin dying. Over-powerful filters create currents that stress or exhaust slow-swimming species. This calculator cuts through misleading box ratings to give hobbyists a practical, buffered GPH target based on their actual tank volume and stocking demands.

When to Use This Tool

Use this calculator before purchasing any new aquarium filter to establish a minimum GPH target before comparing products. Use it when diagnosing unexplained water quality issues to determine whether the existing filter is undersized for the current stocking level. Use it when upgrading tank size to recalculate requirements for the new volume. Use it when adding significant new livestock to an established tank. Use it to compare the real-world adequacy of two or more filters under consideration.

Who is this for?

  • Beginner aquarists choosing their first filter and unsure what GPH rating to look for
  • Hobbyists upgrading to a larger tank and recalculating filtration requirements for the new volume
  • Fishkeepers troubleshooting persistent water quality problems including ammonia spikes, cloudy water, or fish stress
  • Goldfish and cichlid keepers managing high-bioload setups that exceed standard community tank filtration recommendations
  • Planted tank hobbyists balancing CO2 retention against adequate turnover and biological filtration needs
  • Betta and Discus keepers needing to reduce filter current without compromising water quality
  • Marine and reef aquarium hobbyists converting GPH requirements from freshwater standards to saltwater protein skimmer and sump setups
  • Breeders setting up fry tanks and shrimp breeding systems requiring low-flow sponge filtration

A filter is the life support system of your aquarium. It is the single piece of equipment that stands between a thriving, healthy tank and an ammonia-poisoned crash. Getting the flow rate wrong in either direction causes real harm — too slow and dead spots form, ammonia accumulates, and fish suffocate on their own waste; too fast and slow-swimming species like Bettas and Discus are physically exhausted by relentless current until they stop eating and die of stress. This calculator helps you find the correct flow rate for your specific setup.

Who Should Use This Calculator?

Any freshwater or marine aquarium hobbyist choosing a new filter, upgrading an existing one, or troubleshooting water quality problems should use this calculator. It is equally valuable for beginners setting up their first tank who do not yet understand GPH ratings, intermediate hobbyists who suspect their current filter is underpowered, and experienced fishkeepers managing high-bioload setups such as Oscar tanks, African cichlid communities, or heavily stocked breeding systems.

It is also useful for planted tank hobbyists who need to balance filtration against CO2 retention — high turnover rates can gas off dissolved CO2 before plants absorb it, so planted tanks often run at the lower end of the recommended range with a focus on internal circulation rather than surface agitation.

Understanding Turnover Rate

Turnover rate is the number of times your entire tank volume passes through the filter media within one hour. It is the single most practical metric for evaluating whether a filter is appropriately sized for a given aquarium. A filter that turns over the tank volume four times per hour processes every litre of water through its biological, mechanical, and chemical media four complete times during that hour.

Different tank types require different turnover rates based on the bioload — the total quantity of fish waste and uneaten food being introduced to the system each day.

  • 4x Turnover: The practical minimum for lightly stocked, heavily planted tanks, and tanks housing slow-water species such as Bettas, Gouramis, and wild-type Discus. In planted tanks, lower turnover also preserves dissolved CO2.
  • 6x Turnover: The broadly recommended standard for typical community aquariums housing Tetras, Rasboras, Corydoras, and similar species at moderate stocking levels. This is the appropriate starting target for most freshwater setups.
  • 8–10x Turnover: Required for heavily stocked tanks, predatory fish with high metabolic waste output (Oscars, Arowana, large Plecos), and African cichlid systems where aggression management through overstocking further increases bioload. Goldfish tanks fall into this category despite their peaceful temperament — Goldfish are extremely messy feeders producing three to four times the ammonia of a comparably sized tropical fish.

Why Manufacturer Flow Ratings Are Misleading

When a filter box states a GPH or LPH rating, that number is almost always measured under optimal laboratory conditions: an empty filter basket with no media, pumping water horizontally at zero head height. This is as useful as advertising a car's fuel economy measured downhill with a tailwind.

In real-world installation, two factors reduce actual flow dramatically. First, filling the filter with foam pads, ceramic rings, and activated carbon creates flow resistance — dense media reduces throughput by 30% to 50% compared to an empty filter chamber. Second, head height — the vertical distance between the pump and the water surface the return must reach — reduces pressure and flow further. A canister filter installed 60 cm (24 inches) below the tank rim will deliver meaningfully less flow than its box rating suggests, especially under load.

The professional standard in the aquarium hobby is to purchase a filter rated for approximately twice your calculated minimum GPH requirement. This buffer accounts for media resistance, head height losses, and the gradual reduction in flow that occurs as mechanical media accumulates captured particles between cleaning cycles.

The Three Stages of Filtration

Flow rate alone does not determine filter effectiveness. Filtration quality depends equally on how the water is processed as it moves through the media. A complete filtration system addresses three distinct processes: mechanical filtration (physically trapping suspended particles — fish waste, uneaten food, and plant debris — in foam or filter floss); biological filtration (converting toxic ammonia from fish waste first to nitrite and then to the less harmful nitrate through colonies of beneficial bacteria living on porous ceramic media); and chemical filtration (removing dissolved organic compounds, tannins, medications, and odours through activated carbon or specific chemical resins).

Of these three, biological filtration is the most critical and the most fragile. Beneficial bacteria colonies require stable flow, stable temperature, and a consistent oxygen supply to remain active. Turning off the filter for more than two hours, using antibiotics in the display tank, or allowing the filter media to dry out will crash the nitrogen cycle and require weeks of re-establishment. Never clean all biological media simultaneously — always rinse ceramic rings in old tank water (never tap water, which contains chlorine lethal to beneficial bacteria) while leaving foam pads in place to maintain the colony.

Balancing Flow Against Fish Needs

High-flow filters are not universally better. Many popular aquarium fish species originate in slow-moving rivers, flooded forest margins, and still pools where water movement is minimal. Bettas, Discus, Angelfish, and most Apistogramma cichlid species experience chronic stress in strong current — they burn excessive energy fighting the flow, their fins become damaged, and their immune systems weaken. For these species, a lower turnover rate (4x) combined with a filter output directed toward the glass to diffuse current is far more appropriate than maximum flow.

Conversely, fast-water species such as Hillstream Loaches, White Cloud Mountain Minnows, and most Danio species actively thrive in high flow and benefit from additional powerheads or circulation pumps supplementing the main filter.

When to Use This Calculator

Use it before purchasing any new filter to determine the minimum GPH or LPH rating you should be looking for. Use it when diagnosing water quality issues in an existing tank — if your filter is undersized for your stocking level, this will make it immediately apparent. Use it when upgrading to a larger tank to recalculate filter requirements for the new volume. Use it when adding significant new livestock to an established tank, as bioload and flow requirements change with stocking density.

Reference Table

Static Aquarium Filter Turnover Rate Calculator Reference

These baseline ratios and decision rules are included directly in the page so visitors can review core guidance before using the interactive calculator.

Scenario Baseline Rule Safety Note
Inputs used by this tool Aquarium Volume, Unit, Stocking Density / Bioload, Planned Filter Type The interactive calculator refines the result from these inputs.
Outputs generated Unit label 1, Unit label 2, True flow, Buy rating The static table gives baseline logic; final value depends on entered values.
Filter TypeTypical GPH RangeBiological CapacityMechanical CapacityEase of MaintenanceBest ForApproximate Cost
Canister Filter100–1500+ GPHExcellent — large media volume, pressurised contactExcellent — multiple mechanical stagesModerate — requires periodic full disassemblyLarge tanks, heavily stocked setups, high-bioload species$$–$$$$
Hang-On-Back (HOB)100–400 GPHGood — limited by media tray volumeGood — but water can bypass clogged mediaEasy — media cartridge replacement, accessible trayCommunity tanks, beginner setups, quarantine tanks$–$$
Sponge Filter10–60 GPHExcellent for size — vast surface areaLimited — clogs require frequent rinsingVery easy — squeeze in tank water, no moving parts (except air pump)Fry tanks, shrimp breeding, quarantine, hospital tanks$
Undergravel Filter (UGF)Varies by airstone/powerheadGood — entire substrate acts as bio-mediaPoor — traps detritus under plates, difficult to cleanDifficult — requires substrate vacuuming and plate removalLightly stocked tanks; largely obsolete in modern hobby$
Fluidised Bed FilterVariable — inline installationOutstanding — sand in constant motion maximises bacteria contactNone — mechanical pre-filtration required upstreamModerate — requires pre-filter and periodic sand inspectionHigh-demand biological filtration supplement in large systems$$–$$$
Wet/Dry Sump Filter200–2000+ GPHOutstanding — trickle tower maximises oxygen exposureGood — sock or pad pre-filtration at water entryModerate — sump access required, regular sock cleaningLarge predator tanks, marine systems, heavily stocked display tanks$$$–$$$$
Troubleshooting Guide
1 Aquarium water is persistently cloudy despite running the filter

2 Ammonia is detectable in an established tank that has been running for months

3 Filter flow rate has dropped noticeably over time

4 Fish are showing stress near the filter outlet — being pushed around by current

Glossary of Terms
Turnover Rate
The number of times the entire volume of aquarium water passes through the filter media in one hour. Expressed as a multiplier (e.g., 6x turnover for a 50-gallon tank = 300 GPH). The primary metric for evaluating whether a filter is appropriately sized for a given aquarium and bioload.
GPH (Gallons Per Hour)
The standard unit of filter flow rate used in the United States and other countries using imperial measurements. Represents the volume of water the filter pump moves through the media in one hour under specified conditions.
LPH (Litres Per Hour)
The metric equivalent of GPH, used in most countries outside the United States. 1 GPH equals approximately 3.785 LPH. Filter specifications in Europe, Asia, and Australia are typically expressed in LPH.
Head Height
The vertical distance between a filter pump and the water surface level of the aquarium the return must reach. Head height creates back pressure that reduces actual flow rate below the manufacturer's rated GPH. A critical factor when selecting canister filters installed below tank level.
Nitrogen Cycle
The biological process by which toxic fish waste ammonia (NH3) is converted by beneficial bacteria to nitrite (NO2-) and then to the less toxic nitrate (NO3-). The filter's biological media hosts the bacteria colonies that perform this conversion. An established nitrogen cycle is essential for any inhabited aquarium.
Biological Filtration
The filter stage where beneficial bacteria (primarily Nitrosomonas and Nitrospira species) colonise porous media surfaces and convert ammonia and nitrite through the nitrogen cycle. Biological filtration is the most critical filter function and the most sensitive to disruption. Never clean biological media with tap water.
Mechanical Filtration
The filter stage where solid particles — fish waste, uneaten food, plant debris, and suspended sediment — are physically trapped in foam pads, filter floss, or filter socks before they decompose into ammonia. Mechanical media should be cleaned regularly to maintain flow without disrupting biological media.
Chemical Filtration
The filter stage where dissolved organic compounds, tannins, medications, chlorine, and odour-causing molecules are removed from the water column through adsorption onto activated carbon or specific chemical resins. Optional in most community setups but valuable after medication treatment and for maximum water clarity.
Bioload
The total volume of organic waste (primarily ammonia from fish excretion and decomposing food) being introduced to the aquarium system at any given time. Bioload varies by species, fish size, feeding frequency, and stocking density. High-bioload setups require proportionally higher filter turnover rates.
Dead Spot
A zone of poor or stagnant water circulation within an aquarium, typically behind tall decor, in tank corners distant from the filter outlet, or at the substrate surface in large tanks. Dead spots accumulate detritus and develop low oxygen levels, producing localised ammonia buildup that can stress bottom-dwelling fish even when overall water quality appears acceptable.
Impeller
The small rotating magnetic component inside a filter or pump that moves water by centrifugal force. The impeller is the mechanical heart of any powered filter. It is the most common failure point in aquarium filters and should be inspected, cleaned of calcium deposits and debris, and replaced if worn approximately once every 12–18 months.
Beneficial Bacteria
The microorganisms — primarily Nitrosomonas and Nitrospira species — that colonise filter media surfaces and perform the nitrogen cycle. These bacteria require constant oxygenated water flow to survive. They are established through the tank cycling process and are killed by chlorine, certain medications, extreme temperature changes, and filter shutdown exceeding 4–8 hours.
Canister Filter
An external filter housed in a sealed pressurised canister that forces water through multiple stacked media chambers under pressure. Canister filters offer the largest biological media volume and most thorough water-to-media contact of any filter type, making them the preferred choice for large, heavily stocked, or high-demand aquariums.
HOB Filter (Hang-On-Back)
A common, affordable filter style that hangs on the rear tank rim and uses a pump to lift water into an open media tray before returning it via a waterfall outlet. HOB filters are easy to maintain and adequate for most community tanks but provide less media volume than canister filters and allow some water bypass around clogged media.
Sponge Filter
An air-pump driven filter using a porous foam cylinder for simultaneous mechanical and biological filtration at very low flow rates and minimal current. Ideal for fry tanks, breeding setups, shrimp tanks, and hospital tanks. Inadequate as sole filtration for heavily stocked adult fish tanks due to limited mechanical capacity.
Scientific References
  1. Aquarium Science — Biological Filtration and the Nitrogen Cycle
  2. The Complete Fishkeeper
  3. Tropical Fish Hobbyist — Understanding Aquarium Filtration

Frequently Asked Questions

What GPH filter do I need for my aquarium?
Multiply your tank volume in gallons by the appropriate turnover multiplier for your stock level. For a lightly stocked or planted tank: 4x. For a typical community tank: 6x. For heavily stocked or messy fish: 8–10x. Then add a 50% buffer to account for the real-world flow loss caused by filter media resistance and head height. A 55-gallon community tank needs a minimum calculated flow of 330 GPH (55 × 6), so you should buy a filter rated for at least 495–660 GPH to ensure adequate real-world performance. The calculator above applies these multipliers automatically.
What is filter turnover rate and why does it matter?
Turnover rate is how many times the entire volume of your tank water passes through the filter in one hour. It matters because the nitrogen cycle — the biological process that converts toxic fish waste ammonia into less harmful nitrate — depends on consistent water contact with the beneficial bacteria living in your filter media. Too little turnover means water spends too long between filtration cycles, allowing ammonia and nitrite to accumulate to dangerous levels. Too much turnover can create excessive current that stresses slow-swimming species and strips dissolved CO2 from planted tanks.
Why does my filter's actual flow rate seem lower than what the box says?
Manufacturer GPH ratings are measured under ideal laboratory conditions — an empty filter basket with no media installed, pumping water at zero head height (no vertical lift). In real aquarium use, two factors reduce this significantly. Filling the filter with foam pads and ceramic rings creates flow resistance that cuts throughput by 30–50%. Additionally, head height — the vertical distance between the pump and your tank's water surface — reduces pressure and flow, especially in canister filters installed below tank level. This is why experienced aquarists size filters at 1.5 to 2 times the calculated minimum.
Can I have too much filtration or too much flow?
You can never have too much biological filtration capacity — more beneficial bacteria surface area is always an advantage. However, you absolutely can have too much water flow (current). Strong currents physically exhaust slow-swimming fish with flowing fins such as Bettas, Fancy Guppies, Discus, and Angelfish. Signs of excessive flow include fish constantly swimming against the current, refusing to eat, congregating in corners or behind decor to shelter, and progressive fin damage. If your filter is too powerful, redirect the outlet nozzle toward the glass to diffuse the current, or add a spray bar to distribute flow across a wider area. A pre-filter sponge on the intake also reduces flow while adding mechanical filtration.
What is the difference between a canister filter and a hang-on-back (HOB) filter?
Canister filters and HOB (Hang-On-Back) filters can have similar GPH ratings, but canister filters are significantly more effective at biological filtration because they force water through multiple deep layers of dense media under pressure, with no opportunity for water to bypass the media. HOB filters are partially open to the air and allow some water to channel around clogged media rather than through it. Canister filters also hold substantially more media volume for their footprint, supporting larger beneficial bacteria colonies. However, HOB filters are easier to maintain, less expensive, and entirely adequate for most community tanks. The choice should be based on bioload and maintenance preference rather than flow rate alone.
How do I know if my filter is too small for my fish?
The most reliable indicators of an undersized filter are: ammonia or nitrite readings above zero in an established tank (any detectable level is dangerous), persistent cloudy or yellow-tinted water that does not clear within 24 hours of a water change, unpleasant odours from the tank, fish gasping at the surface due to oxygen depletion in stagnant zones, and unexplained fish deaths or chronic disease outbreaks. If you measure zero ammonia and zero nitrite consistently, your filtration is adequate for your current bioload — regardless of whether the filter seems small for the tank. Water testing is the definitive diagnostic tool.
What is head height and how much does it affect my canister filter's flow?
Head height is the vertical distance the filter pump must push water upward from the pump to the tank's water surface. Gravity resists this effort, reducing flow rate proportionally to the height. Most canister filter manufacturers publish a head pressure curve showing flow rate at various head heights. As a practical rule, expect to lose approximately 10–15% of rated flow per 30 cm (12 inches) of head height. A canister filter installed 90 cm (36 inches) below tank level may deliver 35–45% less than its rated GPH. Always account for head height when selecting a canister filter, especially for tall tank setups or cabinets with deep bases.
How often should I clean my aquarium filter?
Clean mechanical filter media (foam pads, filter floss) when you notice a significant reduction in flow rate — typically every 2–4 weeks for most tanks. Rinse foam pads in a bucket of old tank water removed during a water change — never use tap water, which contains chlorine and chloramine that kill beneficial bacteria on contact. Biological media (ceramic rings, bio-balls, sintered glass) should be disturbed as rarely as possible — once or twice per year at most — and only rinsed lightly in old tank water. Never clean all media at the same time. Chemical media (activated carbon) should be replaced every 4–6 weeks or when it becomes saturated. A well-maintained filter with clean mechanical stages and preserved biological media will outperform a neglected oversized filter every time.
What is the nitrogen cycle and why is my filter central to it?
The nitrogen cycle is the biological process by which toxic ammonia — produced continuously by fish through gill excretion and waste decomposition — is converted to progressively less harmful compounds by bacteria. Ammonia (NH3/NH4+) is converted to nitrite (NO2-) by Nitrosomonas bacteria, then nitrite is converted to nitrate (NO3-) by Nitrospira bacteria. Nitrate accumulates and is removed through regular water changes. Your filter's biological media — ceramic rings, sintered glass, or sponge — provides the enormous surface area where these bacterial colonies live. Without adequate flow delivering a constant oxygen supply across this media, the bacteria cannot remain active and the cycle collapses. This is why the biological stage of your filter is more important than its raw GPH rating.
How do I cycle a new filter quickly and safely?
The fastest way to cycle a new filter is to seed it with established biological media from an existing healthy tank. Place a handful of used ceramic rings or a section of mature sponge from a cycled filter into your new filter alongside fresh media — the transferred bacteria colonise the new media rapidly. Without seeding, a fish-in cycle using pure ammonia dosing (1–2 ppm target) or a fishless ammonia source takes 4–6 weeks for the bacteria colonies to establish. During cycling, test ammonia and nitrite daily. The cycle is complete when you can dose ammonia to 2 ppm and see it convert fully to nitrate within 24 hours with zero ammonia and zero nitrite detectable.
Do planted tanks need less filtration than non-planted tanks?
Heavily planted tanks with dense, fast-growing plant mass do partially substitute for some filtration functions — plants absorb ammonia and nitrate directly as fertilizer, which reduces the nitrogen load on the filter. However, this does not eliminate the need for adequate filtration. Biological filtration remains essential because plant uptake is not fast enough to prevent ammonia spikes immediately after feeding or during lights-off periods when plants stop actively absorbing nutrients. The practical difference is that heavily planted tanks can comfortably operate at 4x turnover (rather than 6x) because the plant mass handles a portion of the bioload — and because high flow rates gas off dissolved CO2 in CO2-injected setups. Lower turnover in planted tanks should be compensated by maximising biological media surface area in the filter.
What is the best type of filter for a goldfish tank?
Goldfish are among the most demanding freshwater fish in terms of filtration requirements. They are omnivorous scavengers that eat constantly, produce three to four times the ammonia waste of a comparable tropical fish, and have no stomach — food passes through their digestive system rapidly, increasing organic waste output further. For goldfish, use a filter rated for 8–10x turnover minimum. Canister filters with large biological media chambers are the most effective choice for fancy goldfish. Sponge filters are suitable for quarantine or breeding tanks but inadequate as primary filtration for adult goldfish. A useful starting point: a single fancy goldfish in a 20-gallon tank needs a filter rated for at least 200 GPH after applying the media resistance buffer.
Can I run two smaller filters instead of one large one?
Yes — running two filters is a strategy experienced hobbyists actively recommend. Dual filtration provides full redundancy (if one filter fails, the other maintains the nitrogen cycle while you source a replacement), distributes flow around the tank more evenly to eliminate dead spots, and allows maintenance of one filter at a time without disrupting the entire beneficial bacteria colony. When running dual filters, clean them on alternating schedules — clean one filter, wait three to four weeks, then clean the other. This ensures the biological media is never fully disrupted at the same time. For an aggressive or high-bioload tank, two separate HOB filters covering opposite ends of the tank often outperform a single large canister filter of equivalent total GPH.
What is a sponge filter and when should I use one?
A sponge filter is an air-pump driven filter consisting of a porous foam cylinder that traps particles mechanically while providing enormous surface area for biological bacteria colonies. They have very low flow rates and produce almost no current, making them ideal for fry tanks and breeding setups (where strong currents can harm newborn fish), hospital and quarantine tanks, shrimp breeding tanks (where even small intake currents can trap and kill shrimp), and as supplemental biological filtration in any setup. Their limitation is mechanical filtration capacity — they require frequent squeezing in tank water to clear trapped particles and cannot handle the waste output of heavily stocked adult fish tanks as a sole filtration system.
How do I reduce the current from a filter that is too powerful?
Several effective methods exist. Directing the filter's output return nozzle toward the glass wall diffuses the flow across the surface rather than creating a direct current stream. Attaching a spray bar to the return outlet distributes flow horizontally across the tank length at low velocity. Placing a pre-filter sponge over the intake reduces flow rate by adding resistance at the input stage. For HOB filters, an aquarium safe baffle made from a water bottle or sponge placed over the outflow waterfall reduces surface turbulence. Reducing the pump speed via a flow control valve is the most precise method on canister filters that include one. All of these approaches reduce surface agitation without reducing the biological filtration capacity of the media inside.
What happens if I turn off my filter for an extended period?
Beneficial bacteria in filter media require a constant supply of oxygenated water to survive. When flow stops, oxygen depletes rapidly within the media and the bacteria begin dying. In warm water (above 25°C / 77°F), significant bacterial die-off can begin within 2–4 hours of the filter stopping. After 8–12 hours without flow, a substantial portion of the colony will have died, causing an ammonia spike when the filter restarts. Never turn off your filter overnight to reduce noise. If a power outage exceeds 4 hours, manually aerate the tank and pour tank water through the filter media periodically to prevent complete colony loss. Filters should only be temporarily stopped during certain in-tank treatments — always restart them as quickly as possible.
Does activated carbon actually do anything and do I need it?
Activated carbon performs chemical filtration by adsorbing dissolved organic compounds, tannins, certain medications, chlorine, and odour-causing molecules onto its highly porous surface. It is effective at polishing water clarity and removing discolouration caused by driftwood tannins. However, it does not remove ammonia, nitrite, or nitrate, and its adsorption capacity is finite — once saturated (typically within 4–6 weeks), it stops working and can potentially leach accumulated compounds back into the water. For a well-maintained tank with regular water changes, activated carbon is optional rather than essential. It is most valuable after medication treatment (to remove drug residues before reintroducing invertebrates or sensitive species) and for display tanks where water clarity is a priority.