Fish Medication Dosing Calculator
Dose by actual water volume for common aquarium treatments, at full or reduced strength.
https://calculators.nirajiitr.com/pets/aquarium-dosing-calculator
How it works
The only hard part is the first term. A tank sold as 200 litres holds nothing like 200 litres of water once substrate, rocks, wood and equipment have displaced it, and the gap is commonly 10 to 20 percent — so dosing to the rated volume systematically overdoses. Underdosing is its own problem: a partial dose can leave a parasite population that survives and is harder to shift next time, which is how resistance builds in a system.
dose = actual water volume × concentration per litre × strength- actual volume
- Water in the system, not the tank's rating
- concentration
- Active ingredient per litre for that treatment
- strength
- Reduce for sensitive species
- course
- Total across all treatment days
Worked example
A tank rated at 120 litres, holding about 100 litres once decor is accounted for, treated with praziquantel for flukes.
- 1Praziquantel is dosed at 2 mg per litre
- 2100 litres × 2 mg = 200 mg per treatment
- 3At full strength, the multiplier is 1.0
- 4Single dose, repeated after seven days to catch newly hatched flukes
200 mg now and 200 mg in a week. Dosing to the 120 litre rating instead would have given 240 mg — a 20% overdose every time.
Frequently asked questions
How do I work out my real water volume?
The reliable way is to measure it going in: fill the tank with a container of known volume and count, or note the reading on a water meter. Failing that, take the internal dimensions in centimetres, multiply and divide by 1000 for litres, then subtract roughly 10 to 15 percent for substrate and hardscape and a little more for a heavily aquascaped tank. Note the figure somewhere on the cabinet — you will want it again every time you treat.
Why do I need to remove carbon before dosing?
Activated carbon and similar chemical media adsorb dissolved organics, and most medications are exactly that. Leave carbon running and it strips the treatment out of the water within hours, so the fish get a fraction of the intended dose for a fraction of the intended time — the worst of both worlds. Take it out before dosing, and put fresh media back at the end of the course to clear the residue.
Which treatments are dangerous to invertebrates?
Most of them, and copper-based products especially — copper is lethal to shrimp, snails and corals at concentrations fish tolerate easily, and it binds to substrate and silicone so a treated tank can stay unsafe for invertebrates long afterwards. Malachite green and methylene blue harm scaleless fish such as loaches and pufferfish. Treat in a separate hospital tank wherever the display holds invertebrates.
Will medication crash my filter?
Some will. Antibacterials in particular do not distinguish between a pathogen and the nitrifying bacteria in your filter, and losing that colony mid-treatment gives you an ammonia spike on top of a sick fish. Test ammonia and nitrite daily through any antibacterial course, keep water changes ready, and prefer a bare hospital tank where you can change water freely without disturbing an established display.
Why does oxygen matter during treatment?
Most medications reduce dissolved oxygen, either directly or by affecting the surface film, and a sick fish is already under respiratory stress — gill flukes and many bacterial infections attack the gills specifically. Add an airstone and increase surface agitation for the duration. Fish gasping at the surface during a treatment are far more often short of oxygen than reacting to the drug.