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Aquarium Water Change Calculator

Enter tank volume and change percentage.

How it is calculated

The tank volume is read in US gallons and the change as a percentage of that volume. The tool computes water to remove = tank volume × percent ÷ 100 in gallons, converts it with litres = gallons × 3.785, and gives what stays behind as remaining = tank volume − water removed. All three figures are rounded to one decimal place. The tank volume must be above zero and the percentage must be above 0 and no more than 100; anything outside that range shows no result, because you cannot remove more water than the tank holds. Use the actual water volume, not the rated size of the tank: gravel, rock and decorations displace water, so a 55-gallon tank usually holds a little less.

Worked example

Take a 55-gallon tank and a 20 percent water change. Water to remove: 55 × 20 ÷ 100 = 11.0 gal. In litres, 11 × 3.785 = 41.635, shown as 41.6 L. What stays in the tank: 55 − 11 = 44.0 gal. You would siphon out 11 gallons and refill with 11 gallons of conditioned water at a matching temperature, leaving 44 gallons of old water in the tank.

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Common mistakes

  • Entering the tank size in litres. The volume field is in US gallons, so a 200 L tank must be converted first (about 53 gal); typing 200 asks for a change from a 200-gallon tank.
  • Using the rated tank volume when the tank is heavily decorated. Substrate and rock take up space, so the real water volume, and the amount to remove, is somewhat lower.
  • Typing a fraction such as 0.2 for 20 percent. The field is a percentage, so 0.2 means a 0.2 percent change, which is only a few tablespoons in a 55-gallon tank.
  • Entering a percentage above 100 and wondering why nothing appears. Values over 100 are rejected, since a change cannot exceed the whole tank; exactly 100 is accepted and leaves 0.0 gal.
  • Reading the remaining figure as what to add back. The remaining volume is the old water left in the tank, while the refill is the same amount as the water removed.

About this calculator

A partial water change removes a set percentage of an aquarium’s water — commonly 10–25% weekly — to dilute accumulated nitrates and other dissolved waste that a filter’s beneficial bacteria don’t remove, without disturbing the whole tank’s biological balance the way a full change would. The volume to remove is simply tank volume × change percentage.

Fishkeepers and aquarists follow this routine as a core part of tank maintenance, since nitrate buildup between water changes is one of the leading causes of fish stress and disease in both freshwater and reef tanks.

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Comments (1)

  • Editorial team

    Think of the percentage as a dilution step. A 50% change on a 20-gallon tank removes 10.0 gal and leaves 10.0 gal of old water, so waste such as nitrate roughly halves, provided your tap water is low in it. That is why one big change helps less than people expect compared with steady smaller ones, and why testing your fill water matters. How often do you change, and how big a share each time?

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