Aquarium Medication Dosing Calculator

Overdosing aquarium medications kills fish. Underdosing fails to treat the disease and risks creating drug-resistant strains. Getting the volume right is not optional.

Dose = net water volume × dose rate per gallon

The tricky part is net water volume. The number on the box (10 gal, 55 gal) is gross capacity, measured brim-full and empty.
Substrate, rocks, decorations and equipment take out 10 to 20%, and nobody fills to the brim either, so a “55-gallon” tank usually holds 45 to 50 gallons of water. For anything with a narrow therapeutic window, copper being the obvious one, take the low end of your estimate. Underdosing by 5% is a slower cure; overdosing copper by 20% is a dead tank of shrimp.

If you want that figure properly rather than as a guess, the tank volume calculator works it out from the glass dimensions and the displacement.

Remove activated carbon before treating. Carbon absorbs medication within hours and renders the treatment useless. This is the most common reason a first round of treatment fails.

Some medications reduce oxygen. If the label suggests it, run an airstone during treatment, especially at higher temperatures when dissolved oxygen drops naturally.

Half-dose caution for invertebrates: many medications are toxic to shrimp, snails, and crabs even at doses that are therapeutic for fish. If your display tank has inverts, research the specific medication first. Moving fish to a separate hospital tank for treatment is often safer than medicating the main tank.

Quarantine tanks prevent this problem entirely. A 10-gallon bare-bottom tank with a sponge filter is inexpensive and allows you to treat new fish before they enter your display tank, and sick fish without exposing the rest of the tank to medication stress.

After treatment, run fresh activated carbon for 24–48 hours to remove medication residue before it affects your biological filter.