Dechlorinator Dosage Calculator
Find out exactly how much water conditioner to add for a water change or a new tank. Pick a product preset or type in any label dose, and get the answer in mL, drops and teaspoons.
How to use it
- Find out what your tap water contains. Your water utility's annual water quality report says whether it uses chlorine or chloramine. If you are not sure, dose for chloramine.
- Enter the volume of new water, not the tank size, for a water change. For a brand-new tank, enter the full volume.
- Choose your product or enter the dose printed on your bottle. Labels change between formulas and countries, so if your bottle says something different from our preset, use "Other" and type in your label.
- Add the conditioner to the new water before it goes into the tank, or as you fill, as your label directs.
Product label doses used
| Product | Chlorine | Chloramine |
|---|---|---|
| API Tap Water Conditioner | 1 mL per 10 US gal (38 L) | 5 mL per 30 US gal (114 L) |
| Seachem Prime | 5 mL per 50 US gal (200 L) | Same; double dose allowed for exceptionally high chloramine |
These doses come from the manufacturers' own product pages. Some retailer listings for API show 1 mL per 20 gallons, which matches an older or different bottle, so always compare against the label on your bottle. Seachem gives both "50 US gallons" and "200 L"; since 50 US gallons is actually 189 L, the calculator uses the US gallon figure, which is slightly more generous in liters.
Formulas used
US teaspoons = mL ÷ 4.929
Volumes are converted with 1 US gallon = 3.785 L and 1 UK gallon = 4.546 L. One US teaspoon is 4.929 mL (NIST). Drops assume a standard 20 drops per mL; real droppers vary.
Worked example
You are changing 40 US gallons of water that contains chloramine, using API Tap Water Conditioner.
The chloramine dose is 5 mL per 30 gallons: 5 × 40 ÷ 30 = 6.67 mL. That is about 133 drops or 1.35 US teaspoons.
With Seachem Prime, the same 40 gallons needs 5 × 40 ÷ 50 = 4 mL.
Chlorine vs chloramine
Water utilities disinfect tap water with either chlorine or chloramine. Both are toxic to fish because they damage gill tissue and, in the case of chloramine, enter the bloodstream through the gills and reduce the blood's ability to carry oxygen.
Chlorine slowly escapes from standing water. Chloramine is chlorine bonded to ammonia. It is much more stable: standing, boiling or adding salt will not remove it, and it can last for weeks. The US EPA advises fish owners to neutralize or remove chloramine from aquarium and pond water.
Most conditioners work with a reducing agent such as sodium thiosulfate, which converts chlorine into harmless chloride and is itself oxidized to sulfate. When chloramine is broken apart this way, the chlorine part is neutralized but the ammonia is released. That is why chloramine needs a higher dose of some products, and why many conditioners also bind or detoxify ammonia. In a cycled tank, the filter bacteria handle the small amount of ammonia left.
Frequently asked questions
How much Seachem Prime do I use?
Seachem's label says one capful (5 mL) for each 200 L, or 50 US gallons, of new water. That is 0.1 mL per gallon. For exceptionally high chloramine, Seachem says a double dose may be used safely.
Should I dose for the whole tank or just the new water?
For most conditioners, dose for the volume of new tap water you are adding. Seachem notes that if you add Prime directly to the aquarium, you should base the dose on the whole aquarium volume. Check your product's label.
What is the difference between chlorine and chloramine?
Chlorine is a disinfectant that slowly escapes from standing water. Chloramine is chlorine bonded to ammonia, which is more stable and does not leave by standing, boiling or adding salt. When a conditioner breaks chloramine, ammonia is released, so many products also bind or detoxify ammonia.
Can I overdose water conditioner?
Small overdoses are usually harmless, and some labels allow extra doses in specific situations, such as Seachem Prime's double dose for high chloramine. Large overdoses can lower dissolved oxygen, so stick to the label and add aeration if you dose extra.
How many drops are in 1 mL?
About 20 drops with a standard dropper, which is the figure used here. Real droppers and liquids vary, so a small syringe is more accurate for tiny doses.
Related calculators
- Water change calculator: how much water to change
- Tank volume calculator: gallons and liters for any tank
- Fishless cycle calculator: start a new tank the safe way
Sources: Prime dosing from Seachem Prime product page. API dosing from API Tap Water Conditioner product page. Chloramine stability and advice to fish owners from the US EPA Region 9 chloramine page (archived) and Pinellas County Utilities. Thiosulfate chemistry from Sodium thiosulfate (Wikipedia) and Pentair AES. Teaspoon and gallon conversions from NIST.
Last updated: October 2026