How often to change aquarium water

About 25% weekly for a typical stocked community tank, with dechlorinated water matched to tank temperature. More during a fish-in cycle, where the water change is the only thing keeping the fish alive. Your nitrate readings tell you whether that default is right for your tank.

Water changes are the single most valuable habit in fishkeeping and the one most often skipped, usually because nothing visibly bad happens when you miss one. The damage is cumulative and invisible, which is a bad combination for motivation.

What a water change is actually for

A cycled aquarium converts ammonia to nitrite and nitrite to nitrate. That is the whole nitrogen cycle and it works well — but note where it stops. Nitrate is the end product, and nothing in your filter removes it.

So nitrate accumulates, indefinitely, in every tank. It is far less toxic than ammonia or nitrite, which is why the accumulation is slow rather than dramatic, but at high levels it stresses fish, suppresses their immune systems and fuels algae. The water change is how it leaves the tank. That is the primary job.

• Removes nitrate, which nothing else in the tank does.
• Removes dissolved organics — the accumulated compounds that make old water yellow and that no test kit measures.
• Replenishes minerals and buffering. Nitrification consumes alkalinity, and a tank that never gets fresh water can drift down in pH until the cycle itself stalls.
• Removes solid waste, if you use a gravel vacuum, before it breaks down into more ammonia.

The schedule

Let nitrate set your real schedule

The 25%-weekly default is a reasonable guess for an average tank. Your tank is not average, and there is a straightforward way to find out what it actually needs.

1. Test nitrate immediately after a water change. That is your baseline.
2. Test again a week later, just before the next change. The difference is how fast your tank accumulates nitrate.
3. If it has risen a lot, change more or more often.
4. If it has barely moved, you can relax slightly — though a fortnight is a sensible outer limit regardless, because nitrate is not the only thing accumulating.

Doing one, start to finish

1. Unplug the heater, and the filter if the water level will drop below its intake. Let the heater sit for ten minutes before refilling — a hot heater meeting cool water can crack.
2. Siphon out about 25%, working a gravel vacuum through roughly a third of the substrate. Rotate which third each week.
3. Go around and under decor, where debris collects out of sight.
4. Prepare the new water: dechlorinate it and get the temperature close to the tank by hand. If you are using a hose-fed changer, dose the tank for its full volume before refilling instead.
5. Refill slowly. Pouring onto a plate or your hand stops the substrate being blasted into a cloud.
6. Plug everything back in and confirm the filter primes.
7. Rinse filter media in the old tank water if it needs it — the bucket you just filled is perfect for this. Never under the tap.

The mistakes

• Rinsing filter media under the tap. Chlorinated water kills the colony you spent weeks growing. This is the single most damaging water change error.
• Changing 100% of the water. Enormous parameter shock, and it strips the bacteria's food supply. Partial changes, always.
• Forgetting the conditioner. You have just added chlorine to a tank full of fish and a filter full of bacteria.
• Cold water straight from the tap. A large slug of cold water is a real shock. Match the temperature by hand.
• Deep-vacuuming the whole substrate every week. Disturbs the population living in it. A third at a time.
• Skipping weeks because the water looks clear. Nitrate is invisible and so are dissolved organics. Clarity tells you nothing.
• Doing a huge change to fix a long-neglected tank. If nitrate is very high, bring it down over several smaller changes across a few days rather than one massive one, so the parameter shift is gradual.

During a fish-in cycle it is different

Everything above assumes a cycled tank on a maintenance routine. If your tank is not cycled and has fish in it, water changes stop being maintenance and become treatment.

Test daily, change 25 to 50% whenever ammonia or nitrite is detectable, and keep going until both hold at zero. This can mean daily changes for weeks. It is the only thing keeping the fish alive while the colony grows. See why do my fish keep dying and how to lower ammonia fast.