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.

Aquarium Medication Dosing Calculator

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.

Tips and 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 safe and an 11 percent overdose. During a treatment, only re-dose for water you actually replace in a change, and always remove chemical filtration like carbon first, since it strips medication straight back out of the water.