What Water Should You Use in a Fish Tank?

What kind of water for a fish tank? For most freshwater aquariums, the answer is tap water treated with an aquarium water conditioner that neutralizes chlorine and chloramine. Aqueon, Tetra, and API all describe untreated tap water as unsafe for fish and conditioned tap water as the normal starting point. Well water can work too, but it should be checked first, because no utility treats or monitors it. Reverse osmosis (RO), deionized (DI), and distilled water are for special cases, such as soft-water species or saltwater mixing, and freshwater fish need minerals added back to them. This guide explains what is in each type of water, what to do before it goes in the tank, and when a different source makes sense.

This guide covers water preparation for home aquariums. It is not veterinary advice and does not diagnose illness. Follow the directions on every water care product, and contact a licensed veterinarian who treats fish if your fish seem unwell.

What Type of Water for a Fish Tank: A Quick Comparison

Tap Water: The Starting Point for Most Tanks

Aqueon’s aquarium water basics article says tap water is usually safe for humans but “can kill aquarium fish,” because municipal water contains chlorine or chloramines, and it tells owners to use a water conditioner both when filling a new tank and when adding replacement water. Tetra’s beginner guide gives the same instruction: “Always add a conditioner when adding tap water to your aquarium to de-chlorinate it.”

The reason is the disinfectant that makes the water safe to drink. The UF/IFAS Extension activity “Chlorine: Friend or Foe?” says that for fish and other aquatic life, “even the slightest trace of chlorine can prove fatal.” API’s page for its Tap Water Conditioner says chlorine and chloramines at high levels destroy fish gill tissue and prevent oxygen uptake, and that even low levels irritate delicate tissue. The University of Illinois College of Veterinary Medicine column on betta care adds that these chemicals can cause immunosuppression or make fish very sick.

Chlorine vs Chloramine

Utilities use one or the other, and the difference changes how you treat the water. The EPA’s chloramines page explains that chloramines are most commonly formed when ammonia is added to chlorine, that they provide longer-lasting disinfection as water moves through the pipes, and that more than one in five Americans uses drinking water treated with chloramines. The UF/IFAS activity notes that many municipal systems add ammonia as a secondary disinfectant, and that the resulting chloramine “is very toxic to fish and other aquatic organisms.”

Pinellas County Utilities in Florida, which uses chloramine, sets out what that means for fish owners in its chloramine FAQ:

• Chloramines “must be removed from any water for fish tanks or ponds,” and they are toxic to saltwater and freshwater fish, aquatic reptiles, turtles, and amphibians.
• Chlorine evaporates when water sits for a few days, but chloramines “may take weeks to disappear,” so letting a bucket stand is not a reliable fix.
• Chloramines can be removed with a water conditioner specifically designed for them or with a granular activated carbon filter, and most home water purifiers are not designed to remove them.
• Removing chloramines chemically releases ammonia, which can also be toxic to fish.

The same FAQ notes that Pinellas switches its disinfectant from chloramines to chlorine for a period each year. Other utilities may do something similar, so read the notices from yours.

Choosing a Conditioner for Your Tap Water

Read the label for the word chloramine, not just chlorine. API says its Tap Water Conditioner removes chlorine, neutralizes chloramines, and detoxifies heavy metals. Tetra says AquaSafe Plus neutralizes chlorine, chloramines, and heavy metals and should be used when setting up a tank and with every partial water change or evaporation replacement. Aqueon’s water conditioner page lists chlorine, chloramine, ammonia, and heavy metals. According to Seachem, its Prime removes chlorine and chloramine and binds ammonia in a form that is harmless to fish while staying available to the biofilter.

Dose by the volume of new water and the directions on the bottle; the amount differs between products and, for some, between chlorine and chloramine. Because chloramine treatment releases ammonia, Aqueon’s FAQ says a single dose of water conditioner “does not always fully neutralize the ammonia,” and advises checking pH and ammonia before adding fish. If you read ammonia after using a binding conditioner, the UF/IFAS fact sheet Ammonia in Aquatic Systems warns that Nessler-method kits can give false high readings, so check which method your kit uses.

Find Out What Else Is in Your Tap Water

Conditioners handle disinfectants and metals, not pH, hardness, or nitrate. Aqueon’s do’s and don’ts recommends having tap water checked for pH, alkalinity, nitrate, and phosphate, and its water quality article notes that some tap water sources are high in nitrate. City customers can also read the utility’s annual water quality report; the EPA’s Consumer Confidence Report page explains these reports. Kits for each reading are grouped in the Water Testing and Care section.

One more habit is worth borrowing from drinking water guidance. The EPA’s page on lead in drinking water advises using only cold water for drinking and cooking, and notes that boiling does not remove lead. That advice is written for people, but filling from the cold tap and then bringing the water to tank temperature is a cautious way to prepare aquarium water too.

Well Water: Check It Before You Use It

Well water usually skips the city disinfectant, but it comes with less information. Aqueon’s water basics article says well water “may have heavy metals,” and that the pH of any source water can be too acidic or too basic for fish. The EPA’s private wells page says private well owners are responsible for delivering safe drinking water to their households, that private wells are not regulated under the Safe Drinking Water Act, and that a U.S. Geological Survey study of 2,100 private wells found about one in five contained at least one contaminant above a human-health benchmark.

Have the water analyzed before you build a tank around it. The University of Nebraska-Lincoln Extension guide to water softening notes that contamination is more likely to go undetected in a private supply, and that laboratory results are more reliable than home kits when a large decision depends on them. The USGS page on water hardness adds that groundwater picks up calcium and magnesium as it moves through soil and rock, so well water can be hard, and it points to the local health agency or water utility for local information. Then read pH, GH, KH, and nitrate yourself, and use a conditioner that detoxifies heavy metals if the analysis or the label suggests it.

If your home has a water softener, keep it in mind. The Nebraska guide explains that a household softener swaps calcium and magnesium for sodium, so softened water is a different thing from soft aquarium water. The guide to softening aquarium water covers the aquarium methods; if your home has a softener, the Nebraska guide notes that a cold line to the kitchen tap is often plumbed to bypass it.

RO, DI, and Distilled Water

Purified water removes nearly everything, including the minerals freshwater fish need. The Nebraska Extension guide to household reverse osmosis lists calcium, magnesium, sodium, nitrate, and many metals among the substances RO removes, and its distillation guide says distillation removes hardness minerals as well as chlorine and chloramines. Not everything goes: the RO guide says dissolved gases such as hydrogen sulfide pass through the membrane. Chloramine needs its own stage; Aquatic Life’s RO/DI FAQ says that for its systems a chloramine removal cartridge is required in place of the carbon cartridge.

Bare water is not a safe home for freshwater fish. Seachem’s page for its Replenish mineral product says a complete lack of minerals causes osmotic stress in fish adapted to mineralized water, and that severe osmotic stress can lead to rapid death. The Illinois veterinary column says distilled water should not be used for bettas because it lacks vital minerals. Aqueon’s water quality article describes RO or DI water with an added remineralizing product as a “more natural and stable way to adjust GH” when tap water is excessively hard or soft. If you use purified water for a freshwater tank, add minerals back with a product made for that job, as its label directs, or blend it with conditioned tap water.

Who uses purified water? Keepers of soft-water species, planted tank owners who want control over hardness, and saltwater and reef keepers who mix salt into it. Detailed trade-offs, including waste water and filter upkeep, are in the guide to RO water for aquariums and the guide to distilled water.

What About Bottled Water?

The Illinois column names bottled water as an alternative to tap water for a betta, describing it as free of the harmful chemicals found in tap water, while ruling out distilled water. Bottled waters vary, so read the label: water sold as distilled or purified falls under the remineralizing advice above. For a large tank, the cost and the number of jugs add up quickly, and conditioned tap water is the more practical route.

Saltwater Tanks Need Treated Water Too

Chloramine is not only a freshwater problem. Pinellas County’s FAQ lists saltwater fish among the animals chloramine harms. API, Tetra, and Seachem each say their conditioners can be used in both freshwater and saltwater. Follow the salt maker’s directions for which water to mix with; the guide to making saltwater covers that process.

Adding New Water to the Tank

Whatever the source, prepare the water before it reaches the fish. Aqueon’s do’s and don’ts says replacement water should be the same temperature as the aquarium water, and Tetra’s AquaSafe directions say the same. Aqueon’s FAQ advises against changing more than 50% of the water at once, because a large change can dramatically shift temperature, pH, and chemistry. Treat the new water in a clean bucket used only for the aquarium, match its temperature with a thermometer, and pour it in slowly.

When Plain Conditioned Tap Water Is Not the Right Fit

• Soft-water breeding projects. Wild-caught or soft-water species may need RO or DI water blended to a target hardness.
• Tap water with a problem reading. High nitrate or phosphate in the source water, which Aqueon flags, works against algae control and water changes.
• Well water with a known contaminant. If an analysis shows a problem, do not rely on a conditioner; use treated or purified water and ask the laboratory or your local health agency about the result.
• Fish that are already sick. Changing water sources is not a treatment. Check ammonia, nitrite, and temperature, and contact a licensed veterinarian who treats fish.

Frequently Asked Questions

What Water Do You Use for a Fish Tank?

For most freshwater tanks, tap water treated with a conditioner that neutralizes chlorine and chloramine, at the same temperature as the tank. Aqueon and Tetra both tell owners to condition tap water every time it goes in, at setup and at each water change.

How Long Should Tap Water Sit Before Adding Fish?

Letting water sit is not a substitute for a conditioner. Pinellas County’s FAQ says chlorine evaporates over a few days, but chloramine may take weeks, and the EPA says more than one in five Americans uses water treated with chloramines. Treat the water with a conditioner as its label directs; it does not need to stand first.

Is Distilled Water Good for Fish Tanks?

Not on its own for freshwater fish. Distillation strips out minerals, the Illinois column says distilled water lacks minerals important for fish health, and Seachem warns that a complete lack of minerals causes osmotic stress. It can serve as a base if minerals are added back or it is blended with conditioned tap water.

Do I Need a Water Conditioner for Well Water?

Often, yes. Well water may not contain chlorine, but Aqueon says it can carry heavy metals, and the conditioners from API, Tetra, and Aqueon each list heavy metals among what they treat. Have the well water analyzed first so you know what you are treating.

Can I Use Water From My Refrigerator Filter or a Pitcher Filter?

Do not count on it to remove chloramine. Pinellas County’s FAQ says most home water purifiers are not designed to remove chloramines, and that high-quality granular activated carbon filters may remove them only with enough contact time. Check the filter maker’s specifications, and use an aquarium conditioner either way.

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