Water Changes in Your Aquarium

Goldfish swimming in tank
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Even when your aquarium tank water looks clear, if you stir up the substrate a bit, you may be shocked at how much detritus is present. When fish are fed, particles of food fall to the tank bottom where they decay. Meanwhile, the food that is eaten is eventually released back into the water as urine or feces, which also adds to the problem of poor water quality.

And yet, waste products are not the only reason water needs to be changed. Trace elements and minerals in the water are important both to the stability of the water chemistry and to your fish. Over time trace elements are either used up or filtered out; if they are not replaced, the pH of the water will drop. Furthermore, a lack of trace minerals will adversely affect the vigor and health of the fish. Giving your fish new water regularly is much the same as giving them vitamins to keep them strong and healthy.

Nitrate and Phosphate Waste

In addition to the debris you can see, other invisible natural waste byproducts called nitrates and phosphates will build up. These put chronic stress on the fish, making them more vulnerable to disease. Elevated nitrates also stunt the growth of young fish and interfere with normal reproduction in adult fish. Nitrates also promote the overgrowth of algae; phosphates have a similar effect. Changing the water is the best way to keep nitrate and phosphate levels low.

Frequency of Water Changes

Water changes should be part of regular aquarium maintenance, but the frequency can vary somewhat, depending on usage. Smaller, heavily stocked tanks will require more frequent water changes than larger, sparsely stocked aquariums.

Change 10 to 15 percent of the water each week. If your tank is heavily stocked, bump that up to 20 percent each week. A lightly stocked tank can maybe get by for two weeks, but this should be the maximum length of time between water changes.

Topping Off for Evaporation

You might think that adding water to the tank is the same thing as changing the water, but that is not the case. Merely adding water does not remove any of the wastes, so do not skimp on the water changes. Simply topping off the tank now and then puts fish at risk of poor health.

Keep an eye on your tank's water line. As the water starts to evaporate, replace it with distilled water. While you can and should use hard water to fill the tanks of certain fish, replacing evaporated water with more hard water can lead to a possibly lethal level of some minerals. Using distilled water replaces exactly what was lost and eliminates the problem of water loss without adding a new one.

illustration of water changes in aquariums
The Spruce / Nusha Ashjaee 


  • Let the water sit for a day; this will dissipate dissolved gasses and allow the pH to stabilize.
  • When doing a water change, vacuum the substrate. Get rid of some of the detritus that is building up.
  • Do not clean the gravel and the filter on the same day. Both harbor beneficial bacterial colonies. Do not disrupt both locations at the same time. Time your filter cleaning so it takes place on a day that you are not changing the water and vacuuming gravel.