Fish live in their own toilet. Everything they eat comes out as waste, and a healthy tank turns that waste into a chemical called nitrate. The filter's helpful bacteria (the ones you grew when you cycled the tank) do this job for you, but nothing in an ordinary tank removes nitrate. It just piles up, week after week.
The only reliable way to lower it is to take out some tank water and put in fresh tap water, which usually has little nitrate in it. That is all a water change is: dilution. The question is how much, and how often.
Welfare and fishkeeping sources agree on the idea but not on one single number. The RSPCA describes a routine siphon of around ten percent, and on another line of the same page mentions weekly changes of around thirty percent, and tells owners to check with their aquatics retailer which suits their tank. Practical Fishkeeping says about a quarter once a week works for most people. A vet-reviewed WebMD guide says 10–15 percent weekly for a smaller tank and around 20 percent for a larger one. Aquarium Co-Op prefers around 30 percent.
So the honest answer is: somewhere between about 10 and 30 percent, roughly every week, depending on your tank. What decides it is how fast nitrate builds up, and that depends on how many fish you have, how much you feed, how big the tank is and whether it has live plants.
Aim to keep nitrate under about 40 ppm (parts per million), or no more than about 40 ppm above what comes out of your tap. Test to find out where you are.
That 40 ppm line comes from Aquarium Co-Op ("generally below 40 ppm is considered safe for most fish") and Practical Fishkeeping (test your tank and your tap water; a gap of more than 40 ppm means change more water, more often). The RSPCA says high nitrate can still harm fish but does not give a number, and some sensitive species prefer lower. If you keep something delicate, ask a specialist or an aquatic vet.
This planner uses one simple rule: after a change, the water is a mix of old and new. If your tank gains a certain number of ppm of nitrate between changes, the level it settles at just before each change is tap nitrate + weekly rise ÷ fraction changed. Type in your tank, pick a stocking level and slide the percentage to see where your nitrate would settle.
Pick a starting guess, then fine-tune below. Goldfish, big fish and lots of feeding push nitrate up faster; live plants and a larger tank slow it down.
15 litres
Changing water is easy to get wrong in a few small ways. Each of these points comes from fish-keeping and welfare sources.
The RSPCA says ammonia and nitrite should both read zero and that test kits for them are essential, not optional. If either shows up in an established tank, the RSPCA advice is large, repeated changes (it mentions half the water every day) while you keep testing. Treat that as an emergency step, and call an aquatic vet or a specialist aquatics shop for advice, especially if fish are gasping, hiding or not eating. This guide is for ordinary freshwater tanks; saltwater tanks, ponds, shrimp tanks and sensitive wild-caught fish have their own rules.
You need little gear to do this well. Look for a liquid or strip test kit that reads nitrate (and ammonia and nitrite), a water conditioner that says it handles chlorine and chloramine, a siphon or gravel vacuum that suits the depth of your tank, and a reliable thermometer. A clean bucket used only for the aquarium (never with soap) is the cheapest, most useful item of all.