FAQ: Water Care for fish

The common advice is to change about 20–30% of the water every week or two. Heavily stocked tanks, goldfish or discus need more frequent or larger changes, and nitrate tests help set the exact routine.

Usually 10–30% of the water is changed. Larger changes, up to half, are sometimes needed when the water is polluted, but the new water must then closely match in temperature and parameters so the fish are not shocked.

Better not, as a full water change abruptly alters pH, hardness and temperature, which is very stressful for fish. Even when the water is badly polluted, several larger partial changes over a few days are safer.

The new water should be as close as possible to the tank water, usually within 1–2 °C. Check with a thermometer before pouring, since water can feel right to the hand even when the difference is larger.

Use a gravel siphon: push the wide tube into the substrate and let water run down the hose into a bucket, carrying the dirt while the gravel stays. Vacuuming fits best with a water change, doing one section of the bottom at a time.

Many siphons have a bulb or valve that starts the flow with a few squeezes. If yours does not, fill the hose with water, cap the end and lower it below the tank. Do not suck water with your mouth, as it is unhygienic.

Hold the siphon just above the sand surface so only the dirt is drawn up, not the sand, or pinch the hose to slow the flow. Do not dig deep into sand, as it is light and quickly pulled into the hose.

No, the fish do not need to come out and it is better not to, since catching them is more stressful than the change itself. Just work calmly, do not pour water directly onto the fish and direct the stream onto an ornament or a plate.

Use a separate bucket reserved for the aquarium that has never held detergents or other chemicals. Two buckets, one for dirty and one for new water, or a hose system that attaches to the tap, are convenient.

Dedicated hose systems let you drain water into the sink and refill straight from the tap without carrying buckets. Set the right temperature before refilling, add conditioner for the whole tank volume and fill slowly so the substrate is not disturbed.

After a change, fish may briefly hide or swim restlessly due to differences in temperature, pH or hardness. If it does not pass quickly, check that you used conditioner and the temperature was right, and in future change smaller amounts more often.

No, topping up only replaces evaporated water, while nitrate, minerals and other accumulated substances stay in the tank and even concentrate. A water change removes part of the old water, so both are needed.

Many people change water before feeding or a few hours after, so the fish eat in peace and the siphon removes leftovers. It is not a strict rule; regular changes matter more.

Yes, ammonia, nitrite and nitrate are invisible, so clear water is not necessarily good water. Regular partial changes remove accumulated nitrate and restore trace elements, even if the tank looks spotless.

Yes, goldfish eat and foul the water a lot, so their tanks often get weekly changes, sometimes 30–50%. A strong filter and enough tank volume also matter so nitrate does not build up too fast.

In a small tank water fouls quickly, so a betta often gets 20–30% changes once or twice a week. The new water must be conditioned and at the same temperature, and pour it slowly so nothing gets stirred up.

Shrimp are sensitive to sudden changes, so they get smaller changes, around 10–15%, and the new water is added slowly, sometimes through a drip line. It is vital that parameters and temperature match the tank water.

Drained aquarium water contains nitrate and organic matter, so many people use it to water houseplants or the garden. Just do not use it if the tank recently had medication or a lot of salt in it.

It is limescale left behind as hard water evaporates. It can be removed with an aquarium scraper, and tougher deposits by drying the glass and rubbing with a vinegar-dampened cloth outside or above the water, then rinsing well.

Yes, regular changes reduce nitrate and phosphate, which feed algae. But lighting duration, feeding and the amount of plants also play a role, so water changes alone do not always solve the problem.