Can you use tap water or borewell water for an aquarium in India?

AquaLog guide · Published 6 October 2026

Is tap water safe for fish?

Not straight from the tap. India's drinking-water standard, IS 10500:2012, requires at least 0.2 mg/L of free chlorine at the consumer's end when a supply is chlorinated. That keeps it safe to drink, but chlorine and chloramine harm fish and kill the bacteria in your filter. The standard also sets a limit for chloramines (4.0 mg/L). We found no Indian water utility confirming that it uses chloramine, but you can't tell which disinfectant is in your water by looking at it.

1. Use a water conditioner (dechlorinator) that says it neutralises both chlorine and chloramine. The University of Florida's extension vet gives the same advice.
2. Dose it for the full volume of new water, following the label, every time: water changes and evaporation top-ups alike.
3. Match the new water to the tank temperature within about 1–2 °C.
4. Pour it in slowly.

Can I use borewell water for an aquarium?

Often, yes, but test it first: groundwater differs from one area, and one borewell, to the next. Nitrate is the reading to check first. In the Central Ground Water Board's annual groundwater quality report, released in December 2024, 19.8% of 15,259 samples collected in May 2023 were above the 45 mg/L drinking-water limit for nitrate, and in Rajasthan, Karnataka and Tamil Nadu more than 40% were. Fishkeepers aim to keep nitrate below about 20–40 ppm, so water like that starts above target before a single fish adds waste, and water changes can't bring nitrate below what is in the source. Our water change calculator takes your source water's nitrate into account and shows how much you would need to change to reach a target, or that you can't.

• Test the raw borewell water for nitrate, pH, GH and KH, not just the tank.
• Check whether your nitrate kit reads nitrate (NO₃) or nitrate-nitrogen (NO₃-N). The 45 mg/L limit is as NO₃, which equals about 10 mg/L as NO₃-N.
• Aerate it before use, for example in a bucket with an airstone. Well water can carry dissolved gases such as hydrogen sulphide, and the University of Florida's extension vet advises aerating it first.
• If nitrate is high, mix it with RO water (below) or use a different source for water changes.

What is the ideal TDS for an aquarium?

There isn't one number, because TDS doesn't say what is dissolved. A TDS meter measures how well water conducts electricity and converts that into an estimate of total dissolved solids, as NDSU Extension explains. Every dissolved mineral adds to it: the calcium and magnesium that make water hard, but also sodium, chloride, sulphate and nitrate. Two waters with the same TDS can have very different hardness, so TDS alone can't tell you whether water suits soft- or hard-water fish.

TDS is still useful for comparisons: whether your RO unit is working, how your tap and borewell water differ, and whether the tank is drifting between water changes. For reference, IS 10500 sets an acceptable TDS of 500 mg/L for drinking water and allows up to 2,000 mg/L where there is no other source.

GH and KH: which tests matter?

GH (general hardness) counts calcium and magnesium; KH (carbonate hardness) counts the carbonates that hold pH steady. Aquarium kits read them in degrees (dGH, dKH). Labs and IS 10500 give hardness in mg/L as calcium carbonate: one degree is 17.8 mg/L, so divide by 17.8. The standard's acceptable hardness for drinking water, 200 mg/L, is about 11 dGH, and its limit where there is no alternative, 600 mg/L, is about 34 dGH. Water that is fine to drink can be far too hard for soft-water fish.

Should you use RO water?

Reverse osmosis removes most dissolved minerals, including calcium, magnesium, sodium and nitrate, according to University of Nebraska–Lincoln Extension. That makes RO water useful for softening hard water or diluting nitrate. On its own, though, it has very little of the minerals fish, plants and shrimp need, and almost no KH to hold pH steady.

• Mix it with tap or borewell water to reach the GH your fish need. The maths is simple: half 20 dGH borewell water and half RO water gives about 10 dGH.
• Or remineralise it with a product made for RO water in aquariums, following the label.
• Make the same mix every time, so water changes don't swing the chemistry.
• Never use the reject water from a home RO unit in the tank. It carries the minerals and contaminants the membrane removed, in concentrated form.
• Don't keep shrimp in pure RO. Cherry shrimp need some hardness to moult; the care guide gives 6–20 dGH.

Which fish suit hard water?

If your water is hard and alkaline, work with it rather than against it. Our list of fish for hard, alkaline water shows every species whose pH range starts at 7 or higher. Beginner choices include guppies, platies and endlers, all pH 7–8.5 and 8–25 dGH. Mollies want it harder still (pH 7.5–8.5, 12–30 dGH) and need at least 90 litres (24 US gal). If your water is soft, see fish for soft, acidic water.

Test your source water once, then your tank regularly. AquaLog records GH, KH, TDS and nitrate alongside temperature and pH for each tank, so you can see whether your water changes are keeping up. Start logging your water, and see aquarium water parameters explained for the numbers to aim for.