Aquarium Forge

Aquarium Water Change Calculator

Enter your tank volume and the percentage you want to change to see exactly how much water to remove, how much to add back, and what remains —in gallons and liters.

Typical weekly changes are 15–30%.

Water to remove (25%)
5 gal / 18.93 L
Fresh water to add
5 gal / 18.93 L
Water remaining after drain
15 gal / 56.78 L
  • Match the temperature of replacement water to the tank to avoid shocking fish.
  • Treat tap water with a dechlorinator, or use RO water remineralized as needed.
  • Saltwater mixing stations and nitrate-driven schedules are planned for a later version.

How It Works

The math is simple: water to remove equals your tank volume times the change percentage, and the same amount of replacement water goes back in. What matters in practice is doing it consistently —regular partial changes dilute nitrates and replenish trace minerals before problems start. This first version covers the core freshwater routine; nitrate driven schedules and saltwater mixing helpers are planned for later versions.

Formula

Water to Remove = Tank Volume × Change %

Water Remaining = Tank Volume −Water to Remove

Example

A 29 gallon tank on a 25% weekly routine: remove 7.25 gallons (27.4 L), add back 7.25 temperature-matched, dechlorinated gallons, leaving 21.75 gallons of old water mixed with fresh.

Important Limitations

  • First version covers freshwater routines; saltwater mixing and nitrate-driven scheduling are coming later.
  • Change percentage is applied to nominal volume —for precision, use actual water volume from the Volume Calculator.

FAQ

How much water should I change in my aquarium?

Most community tanks do well with a 15–0% change weekly. Heavily stocked tanks may need 30–0%, while lightly stocked, well-planted tanks can stretch to every other week. Consistency beats volume —a steady 25% every week is better than a one-off 80% rescue.

How many gallons is a 25% water change?

Multiply your tank volume by 0.25. A 20 gallon tank needs 5 gallons removed and 5 gallons of replacement water; a 55 gallon needs about 13.75 gallons. The calculator converts to liters too.

Should I replace water at the same temperature?

Yes —match the replacement water to the tank within a degree or two. Sudden temperature swings stress fish and can trigger ich. Temperature-matched water makes even large changes safe.

Do I need to dechlorinate tap water?

Unless you use well water confirmed chlorine-free, yes. Treat replacement water with a dechlorinator before it enters the tank, or treat the new water in the bucket first and let it mix.

Can I change too much water at once?

Very large changes (over ~50%) can swing pH, temperature, and dissolved minerals enough to shock livestock. In emergencies (ammonia spike), several staged changes are safer than one massive one.

Sources

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