Water change

A water change is the routine removal of a portion of an aquarium's water, replaced with fresh dechlorinated water, done to dilute nitrate, remove dissolved organic waste and replenish minerals that fish, plants and beneficial bacteria use up over time. Filtration alone cannot remove nitrate the way a water change does, which is why even a well-filtered tank still needs regular changes.

Most planted and fish-only tanks do well with a weekly change of somewhere between twenty and fifty percent of the water, though heavily stocked or heavily fertilized tanks sometimes need more frequent changes to keep nutrients in balance. A gravel vacuum is typically used during the change to pull debris out of the substrate at the same time, which helps prevent waste from building up between changes. New water should be treated with a conditioner to remove chlorine or chloramine and matched reasonably well in temperature before it goes into the tank.

Skipping water changes in a planted tank does not just raise nitrate, it also lets a build-up of other dissolved compounds go unchecked, which can quietly stress fish and shrimp even when nitrate itself looks acceptable on a test kit. A common mistake is changing too much water at once in an established tank, which can shock fish with a sudden shift in water chemistry rather than the gradual change a smaller, more frequent routine provides.