Making Tap Water Safe for Fish: Chlorine and Chloramine

Why tap water needs treating

Public water systems disinfect drinking water to kill germs, using chlorine or chloramine (monochloramine). EPA's Q&A on monochloramine states plainly that chlorine and monochloramine must be neutralized or removed if the water is used in aquariums, for two reasons:

• They harm fish because they enter the bloodstream directly through the gills.
• They can prevent the growth of beneficial bacteria that a healthy tank depends on.

EPA adds that chlorinated and chloraminated water can be used safely in aquariums with products readily available from aquarium supply stores.

Chlorine vs chloramine

Chlorine is the traditional disinfectant. It is volatile, which is why some people let water stand before using it.

Chloramine is chlorine combined with ammonia. Utilities use it because it lasts longer in the distribution system. It is much more stable, and utilities and health departments advise that it does not reliably disappear by standing or boiling. When a conditioner breaks the chlorine-ammonia bond, a small amount of ammonia can be released, which is one reason many conditioners also say they detoxify ammonia, and why a mature biofilter matters.

Find out what your utility uses

• Water quality report. Community water systems publish an annual Consumer Confidence Report, often on the utility's website.
• Ask. Call the utility and ask which disinfectant it uses and whether it ever switches (some change temporarily for system maintenance).
• Test. Total chlorine test kits detect both chlorine and chloramine; free chlorine tests detect only chlorine.

Treating water

1. Choose a conditioner labeled to remove or neutralize chloramine as well as chlorine, if your utility uses chloramine or you're not sure.
2. Dose correctly. Follow the label. Most say to dose for the volume of new water; some for the whole tank. Use our dosing calculator to scale it.
3. Treat before or as you add water. Follow the label on whether to treat in a bucket first or add conditioner to the tank before refilling.
4. Match temperature. Bring new water close to the tank temperature with a thermometer.

Rinsing equipment

Chlorinated water also harms the bacteria living on filter media. When cleaning a filter, rinse the media in water removed from the tank during a water change, not under the tap.

Other things in tap water

Tap water can contain nitrate, phosphate, and varying hardness and pH. Your water quality report lists many of these. Test tap water with your aquarium kit occasionally, so you know what each water change brings in. If tap water carries nitrate, water changes can't take the tank below that level; see the water change calculator.

Well water

Private wells are not treated by a utility and usually contain no chlorine or chloramine, unless you have a treatment system that adds it. Well water can still contain other substances, so test it before using it in an aquarium.

A simple routine

Keep conditioner and a dedicated bucket by the tank, dose every time you add tap water, and recheck your utility's disinfectant once a year or when you hear of changes.

Worked example

You are changing 12 gallons of water. Your conditioner label says 5 mL treats 50 gallons, dosed for the new water. 5 × 12 ÷ 50 = 1.2 mL. If you're refilling straight from a hose into the tank and the label says to dose the whole tank volume when doing so, dose for the full volume instead. The dosing calculator handles either.

Common questions

Can I just let tap water sit out?

That approach is often suggested for chlorine, but water utilities and health agencies note that chloramine does not reliably dissipate by standing. Since many utilities use chloramine, a conditioner labeled for chloramine is the dependable choice.

How do I know if my water has chloramine?

Check your utility's annual water quality report (Consumer Confidence Report) or its website, or call the utility. Utilities sometimes switch disinfectants, so check again if you move or hear of changes.

Does chloramine affect people?

EPA says water with chloramine at regulated levels is safe for drinking and ordinary household use. The special cases are aquariums and kidney dialysis.

How much conditioner do I use?

Follow the product label. Our dosing calculator scales the label dose to your volume.