What Kind of Water for a Fish Tank? Tap, Bottled, RO and Well Water Explained

This guide covers freshwater tanks; for a new tank, start with our fish tank set-up guide.

Each water source type at a glance

Water	What the sources say	Before it goes in
City tap	Chlorine or chloramine (EPA)	A dechlorinator (EPA, makers)
Bottled or spring	Minerals vary by brand (Culligan)	Test it first (Culligan)
Distilled	Should be GH and KH of zero (Swell UK)	In most cases minerals added back (Culligan)
RO or RO/DI	Strips minerals (Swell UK)	Usually minerals back (Culligan); chloramine: RO may not remove it (EPA)
Private well	Not regulated (CDC)	Test it first (Culligan)
Salt-softened	Adds sodium (Culligan)	Usually not as the only source (Culligan); Swell UK: salt softeners “not suitable”
Rain	Can carry pollutants (Culligan)	Test it first (Long’s EcoWater)

Tap water: chlorine, chloramine and a dechlorinator

The US EPA says: “Chlorine and monochloramine must be neutralized or removed if used in aquariums.” Its reason: “Chlorine and monochloramine can be harmful to fish because they directly enter their bloodstream through the gills.” It adds: “Water utilities typically provide consumers with information about disinfection processes.”

• Seachem, a water-care maker, says Prime “removes chlorine, chloramine and detoxifies ammonia, nitrite, and nitrate”.
• API, a water-care and test-kit maker, says Stress Coat works “to remove chlorine and chloramines from tap water and neutralize heavy metals”.
• API’s Tap Water Conditioner page doses chlorine and chloramines differently; follow the label.
• Aqueon, an aquarium-equipment maker: “Always treat tap water with a water conditioner to neutralize chlorine and ammonia before adding it to the aquarium.”

Below: Prime and Stress Coat from our water conditioner roundup, and an Aquatic Life unit with a chloramine cartridge, per Aquatic Life, from our RO/DI unit roundup.

Bottled, spring and distilled water

Culligan says bottled and spring water’s “mineral content varies widely by brand, so it should still be tested before use”. API says: “The level of general hardness in tap and bottled water depends on the source of the water and the treatment processes it has undergone.”

Swell UK, an aquarium retailer, says distilled water “should have a general hardness and carbonate hardness of zero”. Culligan: “In most cases, you’ll need to remineralize distilled water or blend it with conditioned tap water before use.”

RO and RO/DI water: soft, and short of minerals

Swell UK: “Reverse Osmosis is a process that strips tap water of its mineral content”. It warns: “Once you move exclusively to use RO however the pH, GH and KH can drop dangerously low, so use a buffer, even if you want very soft water for Discus, to prevent the pH and KH from crashing.” Culligan says RO water “usually needs minerals and buffers added back before use in a fish tank”, or you can “blend RO water with dechlorinated tap water.”

The EPA, writing about drinking water, says reverse osmosis filters “do not remove monochloramine from drinking water.” Putting those two EPA lines together (our reading): if your supply uses chloramine, check that your RO unit’s maker says it removes it; if not, treat the RO water with a dechlorinator too.

Aquatic Life, an RO-unit maker, warns in its RO Buddie manual: “Before installing this unit, find out if you have Chloramines in your water supply. Contact your local water company for this information. If you have Chloramines, please refer to page 19 prior to installing your RO Buddie. Failure to do so may result in a ruined RO Membrane and poor filtered water quality.” Its page 19: “The Carbon Cartridge removes Chlorine. When Chloramines (Chlorine & Ammonia) are present, a special filter is required.” It also warns: “Do not drink water that has been filtered with a DI resin cartridge.”

Remineralization: follow the label for amounts.

• Seachem’s Equilibrium, for planted tanks, “is ideally suited for use with RO (reverse osmosis) or DI (deionized) water or any mineral deficient water”. Seachem adds: “To maintain KH, we recommend Alkaline Buffer”. Its directions say not to use it when replacing evaporated water.
• Brightwell, a supplement maker, says Remineraliz-P “Reconstitutes Reverse Osmosis, Deionized or Distilled Water for freshwater aquarium use.” Its directions: “Do not add this product directly to an aquarium without first diluting in water as directed.” Its label: “Caution: May cause eye or skin irritation. If in eyes, flush with water for 15 minutes, get medical attention. If on skin, wash thoroughly with water. Keep out of reach of children. Not for human consumption.”

Our tap, RO and distilled water roundup compares five remineralizers.

Well water: test it first

The CDC, on the household side: “Government officials do not regulate, treat, or monitor tap water from private wells.” It says: “Test your well water at least once every year for total coliform bacteria, nitrates, total dissolved solids, and pH level.” and “Contact your health department to find out what other germs or chemicals to test for based on where you live.” and “Use a state-certified laboratory to test your water.” It warns: “If your test results show that there are harmful germs or chemicals in your well water, consider drinking bottled water or another safe source of water.”

Culligan says well water “usually does not contain chlorine or chloramine (unless you are adding it as part of your own treatment)” but “may contain higher levels of hardness, iron, sulfur, bacteria, nitrates or other naturally occurring substances that can affect aquarium water chemistry.”

Salt-softened water and rainwater

Culligan, which sells softeners, says softened water “is usually not recommended as the sole water source for most aquariums”, and: “Because the softening process adds some sodium, it can affect water balance for certain fish species.” Swell UK: “Many domestic tap water softeners use salt (sodium ions,) to soften tap water for drinking, so that’s not suitable for freshwater aquarium fish species.” Culligan adds that softened city water “still needs some filtration to address chlorine or chloramine unless chlorine reduction is built into the softener as well.” To soften tank water instead, see how to soften aquarium water.

Rain: Culligan says “Rainwater can contain pollutants or other variables that make it less predictable for aquarium use.” Long’s EcoWater, a water-treatment dealer: “Testing rainwater quality is essential before using it for your fish.”

Testing new water before it goes in

Aqueon: “Have your tap water tested for pH, alkalinity, nitrate and phosphate to make sure it is acceptable for aquarium use.”

Kits: our freshwater test kit roundup. pH off for your fish? See how to lower pH or how to raise pH.

Putting new water into a running tank

Freshwater tanks; amounts and the siphon: our water change guide.

Frequently Asked Questions

Leaf Home, a water-treatment seller, lists distillation, reverse osmosis and deionization as purification methods and says “the purification process eliminates minerals essential for your fish’s health.”

Culligan says to treat both: “You should always treat tap water and filtered tap water to address chlorine and chloramine before adding to a tank.” Swell UK says a filter jug may not lower general or carbonate hardness, so “test the tap water for GH and KH using aquarium test kits before and after.”

Not if your supply uses chloramine, per the EPA: “Allowing water to sit at room temperature does not remove monochloramine from drinking water.”

No maker or agency we read names a different water source for either. Continuum, a water-care maker, doses bettas like “general freshwater fish” when remineralizing RO water and lists “goldfish and koi, Central American cichlids, and live bearers” separately.

Sources (16)

• US EPA, chloramine Q&A 22 (2009 PDF) and Q&A 29 (PDF) (agency)
• CDC, Testing Well Water (agency)
• Culligan, fish tank water (water-treatment company)
• Long’s EcoWater, aquarium water (dealer)
• Leaf Home, purified water (seller)
• Seachem, Prime and Equilibrium (maker)
• API, Stress Coat, Tap Water Conditioner, GH & KH kit (maker)
• Aqueon, Do’s and Don’ts (maker)
• Aquatic Life, RO Buddie manual (PDF) (maker)
• Brightwell, Remineraliz-P (maker)
• Continuum, Reconstitute RO (maker)
• Interpet, tap water (maker)
• Fluval, AquaClear manual (PDF) (maker)
• Hydor, Theo manual (PDF, linked from hydor.com/products/theo) and support page (maker)
• GloFish, water change sheet (PDF) (fish brand)
• Swell UK, soften aquarium water (retailer)

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