Why use net water volume instead of tank size?

A tank's labelled size is its gross capacity, but substrate, rocks, driftwood, and equipment displace water. Dosing on the gross figure overdoses the fish. Subtracting a displacement percentage gives the real water volume the medication actually treats.

How much displacement should I subtract?

A bare tank loses almost nothing, a planted tank with a few centimetres of substrate loses roughly 10 percent, and a heavily aquascaped tank with deep substrate and large rocks can lose 15 to 25 percent. When unsure, 10 percent is a safe default for a typical setup.

Is it safe to round the dose up?

No. Fish medications, especially copper-based ich treatments and methylene blue, are toxic above their therapeutic level. Always round down or dose exactly. Underdosing slightly is far safer than overdosing, which can kill fish, invertebrates, and beneficial bacteria.

Should I dose for the missing water during a treatment?

When you do a water change mid-treatment, only re-dose the medication for the volume of water you replaced, not the whole tank. Enter the replaced volume as the tank volume to get the top-up dose, keeping the concentration steady.

Does this work for aquarium salt?

Yes. Enter the salt dose in your chosen unit per gallon or litre, such as one tablespoon per five gallons, and the tool scales it to your net volume. Always dissolve salt in a cup of tank water before adding it gradually.

What is the Aquarium Medication Dosing Calculator?

Enter your tank dimensions or volume, account for substrate and decor displacement, and a per-gallon or per-litre dosage to compute the exact medication amount. For fishkeepers treating ich, fungus, or bacterial disease. It runs free in your browser on Gera Tools, with nothing uploaded.

Aquarium Medication Dosing Calculator

Name: Aquarium Medication Dosing Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Dosing fish medication by the number on the tank label almost always overdoses, because substrate, rocks, and equipment displace real water. This calculator finds your true water volume and scales any per-volume dose to it, so treatments land at the therapeutic level rather than the toxic one.

How it works

First the gross volume is found, either from dimensions or a value you enter. For a rectangular tank in centimetres:

gross litres = length × width × height / 1000

Then displacement for substrate and decor is removed to get net water volume, and the label dosage is applied to that:

net volume = gross × (1 − displacement %)
dose       = dosage per unit volume × net volume

Because medication toxicity rises sharply above the therapeutic dose, the result is meant to be dosed exactly or rounded down, never up.

Worked example

A 75 litre tank with 10 percent displacement holds about 67.5 litres of water. A treatment calling for 1 ml per 10 litres needs about 6.75 ml, not 7.5 ml — the difference between a therapeutic dose and an 11 percent overdose. For copper-based ich treatments in particular, that gap can be fatal to invertebrates in the tank.

Why net water volume matters so much

A typical hobbyist setup — sand bed, rocks, plants, filter equipment — can displace 10 to 20 percent of gross tank volume. On a labelled 200-litre tank this is 20 to 40 litres of “invisible” displacement. Dosing by the label size rather than the actual water volume means consistently putting too much medication in, session after session, without realising it.

The displacement percentage is not a fixed number. If you add a large piece of driftwood or move to a deeper substrate, your net volume drops and you should re-run the calculation before the next treatment.

Mid-treatment water changes

When you do a water change during an ongoing treatment, the correct approach is to re-dose only for the volume you removed and replaced, not the entire tank. For example, if you swap 20 litres during treatment of a 67.5-litre system, you add back medication calculated for 20 litres only. The tool accommodates this: just enter the replaced volume as the “tank volume” field to get the top-up dose.

Things to do before dosing

Remove carbon and zeolite. Chemical filtration strips medication out of the water within hours, making treatment ineffective. Swap them for sponge or ceramic filter media during the treatment period.
Check invertebrate compatibility. Many medications, especially copper and formalin treatments, are acutely toxic to snails, shrimp, corals, and other invertebrates even at therapeutic concentrations for fish.
Read the re-dose interval. Most treatments are not one-and-done. The label specifies a dosing interval (for example, re-dose every 48 hours). Use this calculator for each dose, not just the first.
Measure dissolved oxygen. Some medications reduce the efficiency of biological filtration temporarily, which can cause an ammonia spike on top of the disease stress. Running extra aeration during treatment is good practice.