Water changes: how much and how often

Why change water

Even in a tank that's doing well, things you can't see build up. The bacteria turn ammonia into nitrate, but the nitrate stays. So do other waste products, and the minerals that keep pH stable get used up. Clear water can be bad water.

A regular water change removes some of all this and brings in fresh, clean water. It's the easiest and cheapest thing you can do to keep your fish healthy. Mattilsynet writes that changing part of the water often helps when fish are sick. If you change water regularly, you'll get there less often.

How much and how often

Mattilsynet writes that changing part of the water every week is a good routine, for example a fifth of the volume. Many aquarists change a bit more, up to a third.

What suits your tank depends on how many fish you have, how much you feed and how many plants are taking up nutrients. Nitrate is a good guide: if it stays low between water changes, your routine is good. If it rises week by week, change more or more often.

• Small, regular water changes are better than large, rare ones. Big changes all at once can stress the fish.
• Don't change all the water. The bacteria live in the filter and the substrate, not in the water, but a big change in temperature and water values is hard on the fish.

The new water

Tap water is usually fine for fish, but it needs to be prepared:

• Water conditioner. Many water suppliers use chlorine, at least at times, which harms the fish and the bacteria. A water conditioner from the fish store makes it safe in an instant. Dose for the full amount of water you add.
• Same temperature. Mix hot and cold from the tap until the water is about the same temperature as the tank. A thermometer in the bucket is enough. Sudden temperature changes can trigger Ich (white spot).
• The same water every time. If you use something other than tap water, such as RO water mixed with minerals, mix it the same way every time, so the values stay stable.

How to vacuum the gravel

A gravel vacuum is a hose with a wide tube at the end. It sucks up water and light debris, while the gravel stays behind.

1. Turn off the filter and the heater before the water drops below them. A heater left standing in air can crack.
2. Start the siphon with the pump on the hose or by dipping it, and let the water run into a bucket.
3. Push the tube into the gravel and lift it out again, bit by bit. You'll see the dirt swirl up in the tube.
4. Go carefully around plants. With sand, hold the tube just above the surface, or you'll suck up the sand too.
5. Stop when you've taken out enough water, even if you haven't covered the whole bottom. Do the rest next week.

Refill

Pour the new water in slowly, ideally against a rock or a plate, so you don't dig up the substrate or uproot plants. Turn the filter and heater back on once the water is back up.

While the water level is down, it's a good time to wipe the inside of the glass, trim plants and remove dead leaves.

When to change more

• Ammonia or nitrite above 0: change a third to half right away, and keep going every day until the readings come down.
• High nitrate: change more for a while, and check whether you're overfeeding.
• Sick fish: a water change is almost always the first step.

Log your water changes in the app, and you'll see how the readings change over time. More on testing in How to test your water.