How much dechlorinator per water change
Conditioner doses are printed per volume of water. Enter how much tap water goes into the tank and pick your product to get the dose in mL. Not sure how much water to swap? The water change calculator gives you the volume.
What the makers say
Seachem's Prime page gives 1 capful (5 mL) for each 200 L (50 US gal) of new water. It says Prime can go straight into the aquarium, but adding it to new water first is preferred. For small volumes, it notes that each cap thread holds about 1 mL.
For exceptionally high chloramine, Seachem says a double dose may be used safely. To detoxify nitrite in an emergency, it says up to 5 times the normal dose may be used. Its FAQ says the same 5x applies when ammonia or nitrite is above 2 ppm. With chlorine or ammonia low and water above 30 °C (86 °F), it suggests half a dose.
API Tap Water Conditioner lists two rates: 1 mL for each 10 US gal (38 L) of tap water for chlorine and heavy metals, and 5 mL for each 30 US gal (114 L) for chloramines. The bottle has its own dosing cap, and API's page does not say how many mL it holds, so measure in mL if you can.
Dose the new water or the whole tank
Both makers word the dose by the water you add. Seachem says "new water" and API says "tap water to be added". The calculator follows that, so a 25% change in a 40 gal tank is a dose for 10 gal.
Seachem allows dosing the tank directly and says the dose should then be based on the aquarium's volume, without a separate rate. If you treat the tank, enter the tank's full volume above.
Check before you pour
- Ask your water utility whether it uses chlorine or chloramine. The dose differs for API.
- Read your own bottle. Formulas and label text change, and this page only repeats the makers' pages as read in October 2026.
- Rates here are the printed ones for the unit you pick, so 50 US gal and 200 L give the same 5 mL for Prime even though they are not exactly equal.