Aquarium Water Change Calculator

How many gallons a change of a given size replaces, how much dechlorinator that water needs, and what the change actually does to your nitrate reading.

Last reviewed August 2026 · Free · Nothing you enter is stored

The most reliable thing you can do for a tank

Nothing else in the hobby has the same track record. A water change removes dissolved waste, replaces trace elements, resets pH drift and dilutes whatever you have not identified yet, all in one operation, with no way to get it badly wrong as long as the water is dechlorinated and roughly the right temperature. When something is going wrong and the cause is not obvious, changing water is almost always the right first move.

What people argue about is the schedule, not the value of the operation itself. The conventional advice is 25 to 50 percent weekly, and it earns its place: it is simple to remember, it needs no test kit, and it keeps a moderately stocked tank comfortably clean without the owner having to think about it. That is a genuine advantage, because the schedule people actually follow beats the schedule they optimise and then abandon.

There is an honest minority view worth knowing about. Nitrate modelling suggests that a moderately stocked, well-filtered tank takes considerably longer than a week to reach a nitrate level anyone would call problematic, and from that it follows that a weekly 50 percent change is often doing more work than the tank requires. The argument is to test, and to change water when the reading says to. It is a defensible position held by people who keep healthy tanks. It also asks more of the owner, since it only works if the testing genuinely happens. Neither approach is wrong, and the reasoning is worth having in front of you rather than being told which one to follow.

What each change size actually does to nitrate

Dilution is straightforward arithmetic and it is worth seeing laid out, because it explains why some water change routines feel less effective than they should. Starting from a nitrate reading of 40 ppm:

The row that surprises people is the 25 percent one. A 50 percent change halves nitrate, but two 25 percent changes do not — they remove about 44 percent between them. The reason is that the second change is diluting a tank that is already cleaner than the first one found it, so it takes 25 percent of a smaller number away. Repeated partial changes always have diminishing returns, and no sequence of them ever reaches zero.

The practical consequence is that if you are trying to bring down a genuinely high reading, one large change does more than several small ones spread over a week, provided the temperature is matched and the tank is not in a fragile state. And if nitrate is climbing back up quickly between changes, the problem is the input rather than the export, which usually means stocking or feeding.

Dechlorinator doses

Seachem Prime doses at 5 mL per 200 litres, which is 0.1 mL per US gallon. That is a small quantity, and for a 20 gallon change it is 2 mL, so a syringe or the bottle's own cap markings matter more than they look like they should. Prime can be safely multiplied up to five times the standard dose during an ammonia or nitrite emergency, which is the one situation where you dose for the full tank volume instead of just the new water.

API Tap Water Conditioner works out at 1 mL per 10 gallons for chlorine and 5 mL per 30 gallons where the supply is chloraminated. Chloramine is the more stubborn of the two — chlorine will gas off from standing water given time, chloramine will not — so it is worth checking which one your water company uses rather than assuming. Most published municipal water quality reports say so directly.

Seachem Prime doses at 5 mL per 200 litres, which works out to 0.1 mL per US gallon, and can be safely multiplied up to five times in an ammonia or nitrite emergency. API Tap Water Conditioner doses at 1 mL per 10 gallons for chlorine, more for chloramine. Aquarium salt figures are Aquarium Co-Op's therapeutic doses, which are explicitly not a routine additive. The weekly 25 to 50 percent water change is conventional advice; there is a legitimate minority view, backed by nitrate modelling, that a moderately stocked tank does not need anything like that often.

Dose the new water, not the tank

In normal use the chlorine is in the bucket, so that is where the conditioner goes. Treating the new water means the tank never sees chlorinated water at any point, and the bacteria in the filter never take the hit. Dosing the whole tank volume is a different operation for a different purpose, which is binding ammonia or nitrite during a cycling problem, and in that case you want the dose spread across everything in the system.

If you are refilling straight from a hose or a python siphon, the tank effectively is the bucket, so dose for the full volume being added and add the conditioner as the water goes in rather than afterwards.

Temperature, and the day-after problem

New water should be within a couple of degrees of the tank. Cold tap water straight into a tank is one of the more common reasons a fish that looked fine during the water change looks unwell the following day, and the connection is easy to miss because the damage does not show up while you are standing there. Mixing hot and cold at the tap until it feels the same as the tank is usually accurate enough. Filling a bucket the night before with a spare heater in it is better still, and has the side benefit of letting any chlorine gas off.

Gravel vacuuming and the rest of the routine

Dissolved waste comes out with the water regardless of how you siphon it, but solids only come out if you go looking for them. Working the vacuum into the substrate is what turns a water change from a dilution into an export. Do a different section each time rather than the whole bed in one go, which keeps the disturbance to the bacterial population modest and spreads the work out. In a densely planted tank, hovering just above the surface and lifting the loose debris is usually all that is available and all that is needed.

Everything else is habit. Rinse filter media in the water you just siphoned out rather than under the tap, check equipment while your hands are already wet, and give the glass a wipe before the fresh water goes in. None of it is complicated, which is rather the point.

Frequently asked questions