Water changes: how much and how often

The one chore that does the most for your fish.

A weekly change of 25 to 30 percent suits most tanks. Nitrate builds up as the filter does its job, and apart from plants, a water change is the only thing that removes it. Aim to keep nitrate under about 40 ppm. Lower is better.

How to do it

1. Unplug the heater if the water level will drop below it.
2. Siphon water out with a gravel vacuum, lifting waste from the substrate as you go.
3. Treat the new tap water with dechlorinator.
4. Match the temperature to the tank within a couple of degrees.
5. Refill slowly so you don't uproot plants or stress the fish.

When to change more

Heavily stocked tanks, goldfish tanks and any tank with rising nitrate need bigger or more frequent changes. Don't swap all the water and scrub everything at once. A sudden shift in water chemistry does more harm than the dirt did.