How to Do Aquarium Water Changes

With Tips to Help Your Fish Acclimate

Goldfish swimming in aquarium

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Your aquarium water may look clear, but stirring the substrate may reveal a surprising amount of detritus that has settled in the gravel. Changing the tank's water on a regular schedule will help reduce the amount of waste, harmful by-products, and algae that threaten both water quality and fish health.

Why Are Water Changes Important?

When you feed your fish, particles of food fall to the bottom and decay. Meanwhile, food that is eaten is released back into the water as urine or feces. This deposition of waste raises the levels of nitrate and phosphate in the tank, which promote the overgrowth of algae. As the tank becomes dirty and smelly, its oxygen levels fall, and your fish become stressed or even sick as a result.

In addition, trace elements and minerals in the water get used up or filtered out over time. If they are not replaced by an influx of fresh water, the pH of the tank will drop, beneficial biofilter bacteria will die, and your fish will lose vigor.

illustration of water changes in aquariums

The Spruce / Nusha Ashjaee 

Frequency of Water Changes

Water changes should be part of regular aquarium maintenance. The frequency varies, though, depending on the size of the aquarium and the number of fish. Smaller, heavily stocked tanks require more frequent water changes than larger, sparsely stocked aquariums.

A good rule is to change 10 to 15 percent of the water each week. If your tank is heavily stocked, bump that up to 25 percent each week. A lightly stocked aquarium may only require water changes every two to four weeks, but you should monitor it carefully.

It is possible to do too many water changes in an aquarium. The maximum frequency of water changes should be once per day. If you choose to perform daily water changes, be sure to only replace half of the tank's water to avoid disturbing the tank's biological balance and stressing your fish.

 Aquarium Water Change Steps

  1. Ideally, use dechlorinated water for your water change. Let the water sit for a day; this will dissipate dissolved gasses such as any chlorine and allow the water to reach room temperature.
  2. When performing a water change, vacuum your tank's substrate to get rid of some of the detritus that has accumulated since the last water change.
  3. Do not clean the filter when you change your water and vacuum the gravel. Both the filter and the gravel harbor beneficial bacterial colonies. Disrupting both at the same time can harm your tank's healthy biological balance. Instead, clean your filter several days before or after a water change.


Adding water to the tank—or topping it off—to compensate for evaporation does not remove waste, so it is not a substitute for changing the water. Performing regular water changes should not only keep your aquarium healthier but also maintain adequate volume.