Fish care calculator
Aquarium Medication Dosage Calculator | FishZone
Enter your tank size, substrate displacement, and medication type — get an exact safe dose based on true net water volume. Includes scaleless fish half-dose warnings for copper, formalin, and malachite green treatments.
- Scientific Formula
- Reviewed by Experts
- Updated May 2026
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Type
Risk Assessment
Inputs
5
Difficulty
Beginner
Calculation Time
Instant
Quick Answer
Medication labels dose based on gross tank volume (e.g., '55 gallons'), but your tank never holds that much actual water. Substrate, rocks, and hardscape typically displace 10–20% of advertised volume. The formula is: Net Volume = Gross Volume × (1 − Displacement %), then Dose = Net Volume × Medication Rate. For a 55-gallon tank with 2" gravel and heavy rock, net volume is often 43–47 gallons. Dosing as if it were 55 gallons overdoses fish by 17–28%. For scaleless fish (loaches, corydoras, plecos), always apply a 50% dose reduction for copper, formalin, and malachite green treatments. Remove all activated carbon before dosing — it will absorb the medication and render treatment ineffective.
Enter your details
Enter the labeled values below. Results appear without leaving this page.
How to Use This Calculator (Worked Example)
This tool requires 5 key inputs:
- Gross Tank Volume (Gallons): Enter your specific value (e.g., from your tank's test kit or dimensions).
- med-dose-amount: Enter your specific value (e.g., from your tank's test kit or dimensions).
- med-dose-gallons: Enter your specific value (e.g., from your tank's test kit or dimensions).
- scaleless-warning: Enter your specific value (e.g., from your tank's test kit or dimensions).
- Hardscape Displacement: 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
Fish disease creates panic. Owners grab a medication bottle and dose by the advertised tank size, not the actual water volume — leading to overdose that kills the fish they are trying to save. Scaleless fish owners often do not know about the half-dose requirement until after a fatality. This calculator solves both problems: it estimates true net water volume accounting for substrate displacement, and applies species-appropriate dose adjustments before the owner makes an irreversible mistake.
When to Use This Tool
Use this calculator when: (1) A fish shows signs of disease and you are preparing to treat. (2) You are setting up a hospital or quarantine tank and need to dose a new medication. (3) Your tank contains scaleless fish and you need to know the safe reduced dose. (4) You are converting between metric (mg/L) and imperial (mL per gallon) dose instructions. (5) You are treating a pond or irregular-shaped vessel where volume is hard to estimate. (6) You want to verify whether a medication is safe for your invertebrates or plants before adding it.
Who is this for?
- Fish keepers diagnosing and treating ich, velvet, or fin rot for the first time
- Aquarists managing a hospital or quarantine tank for new or sick fish
- Scaleless fish owners (loach, corydoras, pleco, catfish keepers) needing safe dose reductions
- Reef keepers and planted tank owners verifying invertebrate and plant medication safety
- Fish breeders running dedicated treatment tanks with precise volume requirements
- Pond owners calculating medication doses for large and irregular water volumes
- Aquarists upgrading from a small tank who are unsure how dosing scales with volume
Treating fish disease is stressful enough without having to perform complex mathematical conversions in a panic. The most common cause of treatment failure is incorrect dosing — either underdosing (leaving the pathogen alive) or overdosing (killing the fish you are trying to save). Understanding your tank's true water volume is the first critical step to any successful treatment protocol.
How to Use This Calculator
- Enter your gross tank volume: Use the advertised size (e.g., 55 gallons). If you know your actual dimensions, enter length, width, and water height in inches for a more accurate gross volume.
- Enter substrate depth: Measure the depth of gravel, sand, or rock in inches. Each inch of substrate over the full tank footprint displaces approximately 3–5% of volume.
- Select your medication: The calculator applies the correct dose rate (mL/gallon or mg/L) for common treatments. For unlisted medications, enter the dose rate from the label manually.
- Flag scaleless fish: If your tank contains loaches, corydoras, plecos, or other scaleless species, enable the half-dose override for copper, formalin, and malachite green treatments.
- Read your result: The calculator outputs net water volume, the exact dose in mL and drops, and a reminder to remove activated carbon before dosing.
Why Gross Volume is Dangerous
A standard 55-gallon aquarium does not hold 55 gallons of water. Once you account for the glass thickness, a 1-inch headspace at the top, and 2 inches of substrate across the full footprint, the actual net water volume is often closer to 45–47 gallons. If you dose a toxic medication — such as Copper or Formalin — assuming 55 gallons, you have overdosed your fish by roughly 17–22%. In already weakened, sick fish, this margin is frequently lethal.
Real-World Example: 55-Gallon Calculation
Tank dimensions: 48" × 13" × 21" (nominal 55-gallon).
Gross volume: (48 × 13 × 21) ÷ 231 = 56.7 gallons.
Subtract 1" headspace: 56.7 × (20/21) = 53.9 gallons.
Subtract 2" gravel (≈8% displacement): 53.9 × 0.92 = 49.6 gallons.
Subtract moderate rock and hardscape (≈5%): 49.6 × 0.95 = 47.1 gallons.
Net volume to dose against: ≈47 gallons, not 55.
At Seachem Cupramine's label rate of 1 mL per 10 gallons, this means 4.7 mL, not 5.5 mL — a difference that matters significantly for a toxic medication.
Special Considerations for Scaleless Fish
Species without traditional scales — such as Corydoras, Loaches (Clown Loaches, Kuhli Loaches), Plecos, and most catfish — lack the protective barrier that scaled fish possess. They absorb medications directly through their skin at a much higher rate. When using harsh treatments like Malachite Green, Formalin, or Copper-based medications, administer a half-dose (50%) to prevent organ failure in these species. Always verify the manufacturer's label for scaleless fish warnings. If the tank contains a mix of scaled and scaleless fish, always use the scaleless dose for the entire tank.
Invertebrate and Plant Safety
Copper-based medications (Cupramine, Copper Power, CopperSafe) are lethal to all invertebrates — shrimp, snails, crabs, and corals — at any dose. Even trace copper remaining in substrate and silicone after treatment can kill shrimp added months later. Never use copper in a display tank containing invertebrates. Salt-based treatments are generally safe for most freshwater fish but will kill freshwater shrimp and snails. Metronidazole-based treatments (Seachem Metro+, API General Cure) and praziquantel treatments (PraziPro) are generally safe for scaleless fish, invertebrates, and planted tanks at full dose.
Hospital Tank vs Display Tank Treatment
Whenever possible, treat sick fish in a dedicated hospital or quarantine tank rather than medicating the display tank. Reasons include: protecting invertebrates and plants from incompatible medications, preserving beneficial bacteria in the display tank, allowing more precise volume control and dosing, and enabling you to observe and remove a fish without disturbing the main setup. A bare-bottom hospital tank also makes net volume calculation trivial — no substrate displacement to account for.
Carbon and Chemical Filtration
Activated carbon, Purigen, and ChemiPure will adsorb most medications from the water within hours, rendering treatment completely ineffective. Always remove all chemical filtration media before dosing. Leave biological media (sponges, ceramic rings) running — beneficial bacteria are not significantly harmed by most common treatments at label doses.
Reference Table
Static Aquarium Medication Dosage 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 | Gross Tank Volume (Gallons), med-dose-amount, med-dose-gallons, scaleless-warning | The interactive calculator refines the result from these inputs. |
| Outputs generated | Net volume, Final dose | The static table gives baseline logic; final value depends on entered values. |
Common Aquarium Medications — Safety and Target Disease Reference
| Medication / Active Ingredient | Targets | Safe for Scaleless Fish? | Safe for Invertebrates? | Safe for Plants? | Harms Biological Filter? |
|---|---|---|---|---|---|
| Seachem Cupramine (chelated copper) | Ich, velvet, external parasites | No — half dose minimum, still risky | No — lethal at any dose | Generally yes | No |
| Copper Power / CopperSafe (ionic copper) | Ich, velvet, external parasites | No — lethal risk | No — lethal at any dose | Generally yes | No |
| API Ich Cure / Rid-Ich (malachite green + formalin) | Ich, velvet, external fungi | Half dose only | No | No | No |
| Seachem Metro+ / API General Cure (metronidazole) | Internal parasites, Hexamita, bloat | Yes — full dose | Generally yes | Yes | No |
| PraziPro / Praziquantel | Gill flukes, internal worms | Yes — full dose | Generally yes | Yes | No |
| Seachem KanaPlex (kanamycin) | Bacterial infections (gram-negative) | Yes — use caution | Generally yes | Yes | Yes — moderate |
| API Maracyn (erythromycin) | Bacterial infections (gram-positive) | Yes | Generally yes | Yes | Yes — significant |
| Formalin (37% formaldehyde solution) | External parasites, gill flukes, ich | Half dose only | No | No | No |
| Aquarium salt (NaCl) | Minor infections, nitrite reduction, slime coat aid | Use caution — sensitive scaleless spp. | No — lethal to FW shrimp and snails | No for most FW plants | No |
Safety ratings are general guidelines. Always read individual product labels and verify against your specific fish species. 'Safe for invertebrates' means marine and freshwater invertebrates including shrimp, snails, crabs, and corals. When a tank contains both scaled and scaleless fish, always apply the scaleless (lower) dose to the entire tank.
Troubleshooting Guide
1 Fish are gasping at the surface immediately after medication was added
Possible cause: Formalin and some malachite green products deplete dissolved oxygen from the water. This is a known side effect of these treatments, particularly in warm water with low surface agitation.
Immediately increase surface agitation by adding an airstone or pointing a powerhead at the water surface. Do not remove the medication unless fish are showing severe distress (rolling, loss of equilibrium) — in that case, perform a 50% water change immediately. For future doses, always increase oxygenation before adding formalin-based treatments.
2 Medication does not appear to be working after 3–5 days
Possible cause: The three most common reasons for treatment failure are: (1) activated carbon was left in the filter and absorbed the medication, (2) the diagnosis was incorrect and the medication does not target the actual pathogen, or (3) the dose was too low due to incorrect net volume calculation.
First, confirm carbon was removed before dosing. If it was not, perform a 30% water change, remove carbon, and restart the treatment course from day 1. If carbon was removed, reconfirm the diagnosis — ich looks different from velvet, and bacterial infections look different from parasitic ones. Use a magnifying glass and compare to reference images. If the diagnosis was correct, recalculate net volume and consider whether underdosing occurred.
3 Water turned dark brown or green after adding medication
Possible cause: This is normal for malachite green-based products (blue-green color) and some herbal treatments (brown tint). It does not indicate a dosing error.
No action needed — the color will fade over the treatment period and disappear completely after activated carbon is reinstalled post-treatment. If the color is unexpectedly dark and the dose was correct, double-check the concentration of the product (some formulations are more concentrated than standard).
4 Ammonia is spiking during medication treatment
Possible cause: Antibiotic medications (erythromycin, kanamycin, minocycline) can kill or suppress beneficial nitrifying bacteria, stalling or crashing the nitrogen cycle. This is especially common when treating in the display tank.
Test ammonia and nitrite daily during any antibiotic treatment. Perform 25–30% water changes whenever ammonia exceeds 0.25 ppm, taking care to replace the antibiotic lost in the water change with a proportional top-up dose. After treatment is complete, re-establish the cycle using bottled bacteria (Tetra SafeStart, Seachem Stability). This is the strongest reason to treat in a hospital tank rather than the display.
5 Ich is still present after a full 7-day treatment course
Possible cause: Ich has a temperature-dependent life cycle. At cooler temperatures (72°F / 22°C), the encysted stage can last 2+ weeks, meaning a 7-day course may not have covered the full cycle. The medication only kills the free-swimming tomont stage, not the encysted parasite on the fish.
Extend the treatment course to 10–14 days. If possible, raise the water temperature by 2–4°F (within safe range for the species) to accelerate the life cycle and shorten the treatment duration. Always maintain the therapeutic dose consistently throughout — inconsistent dosing allows parasites to survive and reproduce between exposure windows.
6 Fish developed white cotton-like patches after treatment started
Possible cause: Cotton-like growths are a sign of fungal or bacterial infection (columnaris), not ich or velvet. If the medication being used is a copper or formalin treatment targeting external parasites, it will not address fungal or bacterial co-infections — and the stress of medication may be allowing secondary infections to establish.
Add an appropriate antibacterial or antifungal medication if the primary treatment is for parasites. API Pimafix or Seachem KanaPlex are common options. Avoid overlapping two heavily toxic medications — use the lowest effective dose of each and monitor closely. If in doubt, pause and consult an aquatic veterinarian.
Glossary of Terms
- Net Water Volume
- The actual volume of water in an aquarium after subtracting displacement from substrate, rocks, hardscape, and equipment. The correct volume to use for medication dosing. Typically 10–20% less than the advertised gross tank volume.
- Gross Volume
- The total internal capacity of the aquarium if it were filled to the brim with no substrate, decorations, or equipment. The number printed on the box (e.g., '55 gallon'). Using gross volume for medication dosing leads to overdosing.
- Therapeutic Index
- The ratio between the lethal dose (LD) and the minimum effective dose (MED) of a medication. A wide therapeutic index means more safety margin. Formalin and malachite green have narrow therapeutic indices — effective doses and dangerous doses are close together — requiring precise dosing based on net water volume.
- Chelated Copper
- Copper bound to an organic chelating agent to create a more stable, predictable concentration in water. Products like Seachem Cupramine are chelated copper. The chelating agent prevents copper from rapidly precipitating out of solution, maintaining a consistent therapeutic concentration. Still lethal to all invertebrates.
- Ionic Copper
- Free copper ions (Cu²⁺) in solution, as produced by copper sulfate treatments. More immediately aggressive than chelated copper and harder to dose precisely because pH and hardness affect how much stays in ionic form. Requires a copper test kit to verify concentration rather than relying on dose calculations alone.
- Malachite Green
- A synthetic dye used as an antifungal and antiparasitic agent in aquarium treatments (Ich Cure, Rid-Ich, Ich-X). Effective against ich, velvet, and some fungal infections. Has a narrow therapeutic index and accumulates in fish tissue with repeated doses. Half-dose for scaleless fish. Not safe for systems with invertebrates.
- Formalin
- A 37% formaldehyde solution used as an antiparasitic treatment for external parasites, gill flukes, and ich. Has a narrow therapeutic index and depletes dissolved oxygen from the water. Always increase aeration before adding formalin. Half-dose for scaleless fish. Temperature and pH affect efficacy.
- Metronidazole
- An antiprotozoal and antibacterial medication used to treat internal parasites (Hexamita, Spironucleus), bloat in cichlids, and anaerobic bacterial infections. Generally safer for scaleless fish, invertebrates, and beneficial bacteria than copper or formalin-based treatments. Brand names include Seachem MetroPlex and API General Cure (combined with praziquantel).
- Praziquantel
- An antiparasitic medication effective against internal and external monogenean flukes and some cestode tapeworms. Generally considered one of the safer medications for scaleless fish, invertebrates, and planted tanks. Brand names include PraziPro (liquid) and API General Cure (combined with metronidazole).
- Tomont
- The free-swimming, infective stage of the ich parasite (Ichthyophthirius multifiliis) that is released from the fish and must be killed by medication before it re-encysts. The encysted stage on the fish is protected from medication. This is why ich treatment must continue for a full life cycle (7–14 days depending on temperature) even after visible spots disappear.
- Scaleless Fish
- Fish species that lack traditional protective scales, including all loaches (Clown Loach, Kuhli Loach), Corydoras catfish, Plecos, most other catfish, eels, and knife fish. These species absorb copper, formalin, and malachite green directly through their skin at much higher rates than scaled fish, requiring a 50% dose reduction for these medications.
- Hospital Tank
- A separate, dedicated aquarium used to treat sick fish with medication, away from the display tank. Advantages include protecting display tank invertebrates, plants, and beneficial bacteria; easier volume calculation (bare-bottom); ability to monitor sick fish closely; and ability to perform large water changes without disrupting the main setup.
- mg/L
- Milligrams per liter, the standard scientific unit for medication concentration in water. For practical purposes, 1 mg/L is equivalent to 1 ppm (part per million) in freshwater. Used in clinical dosing references; hobbyist product labels typically convert this to mL per gallon for convenience.
Scientific References
- Noga, E.J. (2010). Fish Disease: Diagnosis and Treatment (2nd ed.). Wiley-Blackwell.
- Yanong, R.P.E. (2003). Use of Copper in Marine Aquaculture and Aquarium Systems. UF/IFAS Extension, University of Florida.
- Yanong, R.P.E. (2009). Cryptobia iubilans in Cichlids. UF/IFAS Extension, University of Florida.
- Stoskopf, M.K. (1993). Fish Medicine. W.B. Saunders.