Distilled water fish tank guide: RO, rain, well, bottled and tap water compared

Don't fill a fish tank with pure distilled or RO (reverse osmosis) water on its own. It has almost no minerals and no buffer (KH) to hold pH steady, so pH can swing wildly and fish, shrimp, snails and plants all suffer. Pure water is a great starting point, but you need to add minerals back (remineralise) or mix it with tap water.

For most fish, dechlorinated tap water is the best and cheapest choice. Well water and rain water can work too, but test them first.

Which water is which

Water	What it's like	Use it for fish?
Tap (city) water	Has minerals and usually some KH. Treated with chlorine or chloramine.	Yes, with dechlorinator. See chlorine.
Well water	Often hard, often low in oxygen, may carry dissolved gases, iron, nitrate or hydrogen sulfide. No chlorine (usually).	Often yes, after testing and aerating.
RO water	Tap water forced through a membrane that removes nearly everything.	Only after remineralising or mixing.
Distilled water	Boiled and condensed. As pure as RO.	Only after remineralising or mixing. Fine for topping up evaporation.
Rain water	Very soft, slightly acidic, picks up whatever is in the air and on your roof.	Possible with care; see below.
Softened water (home water softener)	Calcium and magnesium swapped for sodium. pH and KH unchanged.	Many adult fish cope; shrimp, snails and plants need the calcium it removes. Use the unsoftened tap if you can.
Bottled spring or mineral water	Varies hugely by brand. Expensive.	Only if you read the label and test it. Never "flavoured" or "enhanced" water.

When it's a problem

Pure RO, distilled or rain water used alone: pH drifts and can crash overnight. Fish may gasp, clamp fins or die; shrimp fail to molt; snail shells erode.
Switching water source suddenly: a big change in hardness or pH stresses fish. Change gradually over a few weeks.
Untested well water: can carry a "rotten egg" gas (hydrogen sulfide) that is toxic to fish at any detectable level, extra dissolved nitrogen that causes gas bubble disease, very low oxygen, high nitrate, iron, or other metals.
Rain water from a roof: can carry metals from copper or zinc flashing and gutters, bird droppings, dust and moss treatments.

What to do right now

If you've already filled a tank with pure distilled or RO water and fish are in it, test pH and KH. If KH is near zero, replace a quarter to a third of the water with dechlorinated tap water at the same temperature each day for a few days.
Test your tap water for pH, KH, GH and nitrate. Many people switch to bottled or RO water when their tap water is perfectly fine.
If you use well water and smell rotten eggs, or fish gasp after a water change, stop using it until you've aerated and tested it.
Check your fishes' needs. Most community fish adapt to a wide range. Only some species really need very soft water.

What to do next

Remineralising RO or distilled water

Use a remineralising product made for aquariums. Some add only GH (calcium and magnesium), meant for soft-water shrimp. Others add GH and KH, which suit most fish and cherry shrimp.
Aim for some KH. A common guideline is at least 2–3 dKH to keep pH steady. Blackwater specialists sometimes keep less, but that takes experience.
Mix in a bucket, never in the tank. Test GH, KH and pH before the water goes in.
Be consistent. Make each batch the same so every water change matches.

Mixing with tap water

If your tap water is too hard for your fish, blending is the easiest route. Half RO and half tap roughly halves the hardness and KH. Find the ratio that gives the numbers you want, write it down, and use it every time. Dechlorinate the tap portion.

Well water

Get a lab test for nitrate, iron, copper, other metals and hardness. Many health departments offer well water testing.
Aerate before use. Let it stand in a container with an air stone for 24 hours, or splash it in. This drives off hydrogen sulfide, carbon dioxide and extra nitrogen, and adds oxygen.
Limit changes to about 50% unless the water has been aged, because fresh well water can lower oxygen in the tank.
Check for treatment. If your house water goes through a softener, iron filter or chlorinator, take aquarium water from a tap before it.

Rain water

Experts disagree. Some hobby references treat rain water as a clean, free source of soft water. But rain picks up pollution: a 2022 study found PFAS "forever chemicals" in rain worldwide, often above drinking-water guidelines, and roof runoff can carry metals and droppings. If you use it, collect it from a clean roof with no copper or zinc parts, discard the first flush of each rain, store it covered and dark, strain out insects, and remineralise it like RO water. Avoid it near heavy traffic or industry.

Cautions

Shrimp and snails need minerals for their shells; never keep them in unremineralised pure water. See shrimp dying.
Plants need trace elements that RO water lacks; add a fertiliser.
Copper from pipes or flashing is deadly to invertebrates; see copper.

Products that help

GH and KH test kits: tell you what's in your water, not just pH.
TDS meter: a quick check that RO water is pure and that batches are consistent.
Remineralising salts (GH-only or GH/KH): turn pure water into usable aquarium water.
RO unit: worth it if your tap water is very hard or polluted and you keep soft-water species.
Lidded water container with air stone: for ageing and degassing well or tap water.
Dechlorinator: for tap water, and for any blend containing it.

How to prevent problems

Choose fish that suit your tap water; it's far easier than changing the water to suit the fish.
Never change water source in one go.
Top up evaporation with pure water but do water changes with remineralised or tap water, because evaporation leaves minerals behind.
Test new water sources before using them. For adjusting pH, see adjusting pH and pH problems.

Why it matters

Fish and invertebrates need dissolved minerals to balance salts in their bodies and build bones and shells. KH, the carbonate in water, soaks up acids made by fish waste and plants and keeps pH from crashing. Pure water has neither, so it changes quickly with every bit of waste.

When to get help

Ask your local fish store or aquarium club if you're unsure which water suits your fish, and contact your local health department or a water lab for well water testing. Get help fast if fish start gasping or dying after a change in water source.

In the game

Multiple Tank Syndrome doesn't have a water-source card yet. The pH crash card is what happens in real life when pure water with no buffer goes in a tank.