How Aquarium Medication Dosing Works (and Why Most Aquarists Get It Wrong)
Dosing medication, water conditioner, plant fertiliser, or any other chemical additive to an aquarium is fundamentally a unit-conversion problem. The label on the bottle gives you a convenient rounded dose (often "1 teaspoon per 10 gallons" or "1 capful per 5 gallons"), but those rounded instructions hide what is actually being measured: the concentration of the active ingredient in the water column, expressed in parts per million (ppm). When you dose a medication to treat ich, velvet, internal parasites, or bacterial infection, what matters is not the volume of liquid you poured — it is the concentration of the active ingredient your fish is swimming in. Too low and the pathogen survives; too high and you stress (or kill) sensitive species like loaches, tetras, shrimp, and scale-less fish. This calculator does the maths for you so the dose is exact every time.
The core formula the calculator uses is: dose (mL) = (target ppm × tank volume in litres) ÷ (10 × concentration %). Here is why that works. One ppm in water equals one milligram of active ingredient per litre of water (1 mg/L). So if you want a 2 ppm concentration in a 50-litre tank, you need 100 mg of active ingredient added to the tank (2 × 50 = 100). To convert that to millilitres of solution, you use the bottle's concentration percentage. A 5% solution means 5% of any volume you pour is the active ingredient. One millilitre of a 5% solution therefore contains 0.05 mL of active ingredient, which (assuming water-like density of 1 g/mL) is 50 mg. To get 100 mg of active ingredient, you need 100 ÷ 50 = 2 mL of solution. The calculator generalises this for any ppm, any volume, and any concentration.
The calculator also converts the millilitre result to US teaspoons, because most aquarium products are still dosed in teaspoons and capfuls on the bottle. One US teaspoon equals exactly 4.92892 mL (defined by US federal regulation, 21 CFR 101.9(b)(5)(viii)), and we use that precise value rather than rounding to 5 mL. The reason matters: at small doses, a 1% rounding error compounds. A 0.5 mL dose rounded from a 0.493 mL calculation is fine; the same percentage error on a 5 mL dose becomes 0.07 mL, which on a sensitive shrimp tank can be the difference between a therapeutic dose and a lethal one. Showing both mL and tsp lets you use whichever measurement you have a tool for, while keeping the mathematically exact figure visible.
A few practical notes that affect real-world dosing accuracy. First, always dose based on actual water volume, not the nominal tank size on the sticker. A "20-gallon" tank typically holds 16–17 gallons of actual water once you subtract the air gap at the top and the displacement from substrate, rock, and driftwood. Run our Tank Size Calculator first to get the real volume, then use that figure here. Second, if you are dosing into a tank with carbon filtration running, remove the carbon first — activated carbon will adsorb most medications within hours, making the calculated dose ineffective. Third, for invertebrate tanks (shrimp, snails), reduce the dose by 25–50% from the manufacturer recommendation on the first treatment and observe for 24 hours before doing a full dose; many medications are toxic to inverts at concentrations that are safe for fish.
The calculator handles any liquid additive where the concentration is expressed as a percentage of the active ingredient. This includes water conditioners (Seachem Prime at ~5%, API Stress Coat at ~5%), parasite medications (PraziPro at 0.5% praziquantel, ParaGuard at ~5% aldehyde complex), plant fertilisers (Seachem Excel at 1.5% glutaraldehyde, most iron supplements at 1–2% iron), and DIY solutions. For powdered medications where the dose is given in mg/g rather than a percentage, you can convert: 100% pure powder = a "concentration" of 100 in the calculator. If the powder is, say, 50% active ingredient by weight, enter 50 as the concentration and the calculator will give you the correct weight of powder in mg — you then convert mg to grams or fractions of a teaspoon separately using the powder's bulk density.
Finally, a safety note. This calculator is a maths tool — it cannot tell you what concentration is safe for your specific fish, invertebrate, or plant. Always cross-reference the result with the manufacturer's dosing instructions printed on the bottle, and with species-specific care sheets (our Fish Database includes medication-sensitivity notes for each species). When in doubt, dose low, observe for 24 hours, and redose only if the fish tolerates the initial dose. A slightly under-dosed medication that you repeat in 48 hours is almost always safer than a single over-dose that kills your biofilter or your fish.