Aquarium water changes: how much, how often, and why
How much water to change and how often: the arithmetic of dilution, letting your test kit set the schedule, and how to do it without hurting the fish.
What a water change is actually for
An aquarium is a closed box. Fish eat, fish excrete, and the filter turns the ammonia that produces into nitrite and then nitrate. That is the whole nitrogen cycle, and it ends there: nothing in a normal tank removes nitrate in any quantity. It just accumulates, week after week, until something takes it out. Plants take a little. A water change takes a lot.
Nitrate is not the acute poison ammonia and nitrite are, which is why it gets ignored. It is a slow one. Fish living in high nitrate stop growing, breed less, colour down, and lose the immune margin that keeps ich and fin rot at bay, which is why nearly every disease guide on this site starts with “test your water”. The app calls under 20 ppm good and 40 ppm the ceiling; a planted tank is happy a little higher, because the plants are eating it.
A water change does two other things while it is at it. New water brings back the minerals that buffer pH, which the tank consumes slowly, so a tank that never gets changed drifts acidic over months. And it dilutes everything you cannot test for: the organic compounds, hormones and tannins that make old tank water yellow and stale. That is why fish behave differently after a change even when the numbers barely moved.
How much: the arithmetic
A water change is dilution, and dilution is simple. Change a quarter of the water and you remove a quarter of the nitrate. Change half, remove half. The water you leave behind keeps its concentration, so there is no way to take out more than the fraction you swap.
That gives you a rule you can actually use. Test nitrate, decide where you want it, and the percentage follows:
- 40 ppm, want under 20 — a 50 % change lands you at 20. A 25 % change only gets to 30, and next week you are back at 40.
- 30 ppm, want under 20 — a 40 % change gets there.
- 60 ppm or more — two 50 % changes a day apart (60 to 30 to 15) beat one 80 % change, which shifts temperature, hardness and pH all at once and stresses the fish more than the nitrate did.
For a tank with sensible stocking and a working filter, this works out to 20 to 30 % once a week. That is the schedule the app starts every established tank on, and it is the right default for the same reason a bigger tank is: it leaves room for mistakes. A weekly 25 % holds nitrate in a low band that a missed week or a heavy feed cannot push past the ceiling.
The chart is the argument against the “big change once a month” habit. Moving the same total volume in one go instead of four leaves nitrate twice as high on average, and the fish spend most of the month in the top half of that curve rather than the bottom. Small and often beats large and rare — for the fish, and for your back, because a 25 % change on a 100-litre tank is one bucket and ten minutes.
In the app — the water change calculator opens on your tank’s volume: pick a percentage, and it shows the exact litres to swap and where that lands your nitrate against the safe band. When you are done, one tap logs the change in the journal and ticks the task off the schedule, so the record keeps itself.
How often: let the test kit set the schedule
Weekly is the default, not the answer. The answer is in how fast your tank makes nitrate, and the only way to know is to test on a rhythm and watch the number between changes.
Test just before each water change, when nitrate is at its highest. The difference between one week’s reading and the next is your tank’s nitrate production. Then:
- Climbing 5 to 10 ppm a week — your schedule is right. A weekly 25 % holds this in the good band indefinitely.
- Climbing 20 ppm or more a week — the tank is producing faster than the changes remove. Either change more each week, change twice a week, or, more honestly, feed less and stop adding fish. A tank that needs a 50 % change every week to stay under 40 ppm is overstocked, and no schedule fixes that.
- Barely moving — a lightly stocked or heavily planted tank. Every two weeks is fine, and in a planted tank nitrate under 5 ppm is a problem in the other direction: the plants are starving, and algae will win.
Two more triggers that override the calendar. Any ammonia or nitrite above zero in a tank with fish means a change today, whatever the schedule says, and then daily testing until both read zero. And a tank that has not been tested in three weeks is not on a schedule at all, whatever the app claims; it is running on hope.
In the app — every test you log lands on the nitrate trend, and the schedule advisor reads it: when nitrate has climbed 20 ppm across your recent tests, it proposes a tighter water-change cadence with the numbers that justify it, and you apply it with one tap. A tank left untested for three weeks gets a nudge before anything goes wrong.
Doing it without hurting anything
The water you take out is the easy half. The water you put back is where a routine change turns into a disaster, and every one of these is a mistake people make once.
Dechlorinate the new water, always. Tap water carries chlorine, or increasingly chloramine, which does not gas off overnight the way chlorine does. Both kill the bacteria in your filter, and chloramine burns gills directly. Dose the conditioner for the volume you are adding, into the bucket, before it touches the tank.
Match the temperature. Within a degree or two of the tank. A 25 % change with water 5 °C cooler drops the whole tank 1 to 2 °C in a minute, which is exactly the kind of swing that lets ich take hold. A thermometer in the bucket costs nothing.
Vacuum the gravel while you drain. The siphon is already running; point it at the substrate and let it lift the waste that would otherwise rot into next week’s ammonia. Do a third of the tank floor each week rather than all of it: the bacteria live there too.
Leave the filter alone that day. Filter media rinsed in tap water on the same day as a big change removes both halves of the tank’s bacteria at once, and the tank cycles again with fish in it. If the filter needs cleaning, swish it in the bucket of old tank water you just took out, and do it on a different week.
Pour gently. Onto a plate, or against the glass, not onto the substrate. It keeps the water clear and the fish calm.
When to break the rules
A neglected tank. If nitrate reads off the chart because the tank has not been changed in months, do not fix it in one heroic 90 % change. The fish have adapted to that water, and a sudden shift in hardness and pH shocks them more than the nitrate ever did. Change 25 % a day for a week instead. Each step is small; the week adds up to most of the water.
An emergency. Ammonia or nitrite above zero with fish gasping is the opposite case: change 50 % now, because halving a toxin is the only thing that works in minutes, and the risk of the swing is smaller than the risk of the burn.
A cycling tank. No fish, no water changes, unless nitrite has climbed past what your test kit can read. The bacteria live in the filter and the substrate, not in the water, so a change does not reset the cycle; it just is not needed.
A medicated tank. Follow the treatment’s schedule rather than yours, and re-dose for the volume you replace, or the course fails silently.
In the app — the care schedule holds every tank’s water change, test, and filter clean with its own interval, and the reminder arrives at the hour you chose. A change you log through the calculator ticks it off; one you skip shows up as late on the home screen rather than being forgotten.
That is the whole job. Test, change a quarter, dechlorinate, match the temperature, vacuum a little, and let the number tell you when a quarter is not enough. A tank kept that way does not have emergencies; it has Saturdays.
Common questions
Can I change too much water at once?
Yes. The nitrate arithmetic says a 90 % change is best, but the fish have adapted to the old water, and a sudden shift in temperature, hardness and pH shocks them more than the nitrate did. Keep routine changes under 50 %, and bring a neglected tank down in 25 % steps a day apart.
Should I change water during the cycle?
Not unless nitrite climbs past what the test kit can read. The bacteria live in the filter and the substrate, so a change does not reset the cycle; it just is not needed, and it removes the ammonia they are feeding on.
Do I have to vacuum the gravel every time?
A third of the tank floor each week is plenty. The substrate holds bacteria too, and a full deep clean every week removes them along with the waste. In a planted tank, leave the rooted areas alone entirely.
Is tap water safe for a water change?
With a dechlorinator, yes, for almost every tank. Dose the conditioner for the volume you are adding, and choose one that handles chloramine as well as chlorine, because many utilities have switched. Very hard or very soft tap water is a separate question; test it once so you know what you are adding.