Aquarium water change: how much, how often, how
Updated on August 26, 2026 · ~7 min read
In a freshwater community tank you change 20-30% of the water every week, or every two weeks if the load is light. Changing X% of the water lowers whatever is dissolved by X%: from 60 ppm of nitrate a 30% change takes you to 42 ppm, not to zero.
The water change is the maintenance job that solves more problems than any product on sale, and also the one surrounded by the most folklore: some people change 10% every three days, others 50% once a month, and in between sits a pile of rules handed down without the reason.
The reason is one: you are diluting. Everything that builds up in a tank and nothing consumes — nitrate, phosphate, salts concentrated by evaporation, metabolites no test measures — leaves only by replacing water with new water. Everything else follows from that, frequency included.
How much, in a table
| Type of tank | How much | How often |
|---|---|---|
| Freshwater community | 20-30% | weekly |
| Lightly stocked or heavily planted freshwater | 15-25% | every 10-15 days |
| Cycling tank with fish in it | 20-30% | whenever ammonia or nitrite climbs |
| Fish-only marine | 10-20% | every 2 weeks |
| Reef | 10-15% | weekly |
| Shrimp tank | 10-15% | weekly, very slowly |
These are not sacred numbers: they are the starting point. The real check is the nitrate reading before the change. If in freshwater it stays reliably under 25 ppm, the routine works; if it climbs week after week, you need to change more often or more deeply — or feed less, which is the same thing seen from the other side.
The arithmetic, which matters more than it looks
Changing X% of the water lowers everything dissolved by X%, if the new water contains none of it:
value after = value before × (1 − percentage/100)| Nitrate before | 20% change | 30% change | 50% change |
|---|---|---|---|
| 40 ppm | 32 ppm | 28 ppm | 20 ppm |
| 60 ppm | 48 ppm | 42 ppm | 30 ppm |
| 80 ppm | 64 ppm | 56 ppm | 40 ppm |
Two consequences that are always discovered late.
From 80 ppm you do not reach 20 with a single change. It would take changing 75% at once, with the swing in temperature, pH and hardness that entails. Better two or three changes of 30-40% a few days apart.
If the new water already contains nitrate, the sum changes. In many farming regions the tap comes out at 20-40 ppm: in that case the value you converge on is not zero but the tap's. Anyone changing water weekly and never getting below 30 ppm should run the test on the tap water, not on the tank. The full formula is value × (1−f) + new water × f, with f = the fraction changed.
How to prepare the new water
Three things, always.
The chlorine has to go. Chlorine and chloramine in tap water are disinfectants: they kill the filter bacteria. A dechlorinator is needed at every change, dosed on the volume of new water and not on the tank's. Letting the water "stand overnight" evaporates free chlorine but not chloramine, which is now the more widely used: it is not a reliable alternative.
The temperature has to match. Not "lukewarm": the same, checked with a thermometer. A swing of 2-3 °C (4-5 °F) on a 30% change is noticeable, and in already stressed fish it opens the door to disease. Anyone changing a lot of water should do the sum: 30 litres (8 gal) at 20 °C (68 °F) into 100 litres (26 gal) at 26 °C (79 °F) brings the tank to about 24 °C (75 °F).
The hardness has to match. If you cut with RO water, cut in the same proportion as always: changing 30% with much softer water lowers GH and KH by 30%, and a falling KH takes pH down with it. This is the mistake that produces a pH crash after a change done "better than usual".
In marine tanks there is a fourth thing: the salt is dissolved outside the tank, in a bucket with movement, and salinity is checked at tank temperature before pouring — the reference is about 35 ppt in a reef.
The steps
- Switch off the heater and pumps before lowering the level, or you will run them dry (a heater in air is ruined, or cracks the glass).
- Siphon the substrate while removing water: take the detritus out before it becomes nitrate, instead of diluting the nitrate afterwards. In a planted tank only the open areas, without digging among the roots.
- Take the water out into a dedicated bucket, one that has never seen detergent.
- Pour the new water in slowly, ideally onto a plate or against the glass so as not to stir up the substrate. With shrimp and delicate fish, in a trickle or through a length of tubing: they are the inhabitants that suffer most from a fast change in parameters.
- Switch pumps and heater back on, and check that the filter really restarts.
- Watch the fish for a few minutes. Odd behaviour right after a change is nearly always a problem with the new water — temperature, chlorine, hardness — not a coincidence.
The filter is not cleaned together with the water change, and never under the tap. The bulk of the bacterial population lives in the sponges: rinse them in the water taken out of the tank, one part at a time and not all together, and in a different week from the change. Doing both on the same day means disturbing both places where the bacteria live at once.
How often, really: three signals
Nitrate before the change. It is the thermometer of your routine, not a number to look at once a year.
Algae. Nitrate and phosphate high for a long time are the condition in which they thrive. It is not a mechanical relationship — light, movement and maturity count too — but a tank covering itself in green algae on a regular maintenance schedule is telling you something.
The falling water level. Evaporation is not a water change: the water evaporates and the salts stay, so topping up concentrates GH, KH and (in marine tanks) salinity. Top-ups are done with pure RO water; a water change is a different thing.
What not to do
- Do not change all the water "for a deep clean". An emptied tank loses part of the substrate bacteria and restarts with a mini-cycle.
- Do not change water while cycling a tank with no animals in it: you are taking food away from the bacteria and the cycle gets longer. With fish in the tank the priority flips.
- Do not skip the dechlorinator "because it is only 20%". The chlorine goes through the filter all the same.
- Do not use hot water from the boiler: it carries dissolved metals and residue from the system. Mix cold with boiled, or heat it separately.
How Gurglee does it
Gurglee shows the effect of a change before you make it: on the parameter's page you enter the percentage and the app says where the value would land, on that tank's volume, with the option of declaring how much the new water contains — the piece missing from every generic table. It also tells you which percentage it would take to get back into the band, and when one change is not enough it says so instead of rounding. The change is logged as a recurring activity, with its reminder and the history of when it was actually done.
Related guides
High nitrate: how to lower it for real
Why nitrate climbs in an aquarium, which values to hold in freshwater and in a reef, and the methods that actually lower it — from water changes to plants.
Water hardness: GH and KH explained simply
GH and KH are not the same thing: what they measure, which values fish and shrimp need, and how to raise or lower them without doing damage.
Ideal water parameters chart by type of tank
The reference values of every water parameter, for freshwater, planted, brackish, marine and reef tanks, in one chart you can come back to.
Aquarium algae: identify them by colour and get rid of them
How to recognise aquarium algae by colour and texture — brown, green, hair, black brush, cyanobacteria — and what to do about each of them.