Tank volume
For a rectangular tank, length × width × height in inches ÷ 231 gives US gallons; in centimeters ÷ 1,000 gives litres. Substrate, rock and decorations take up space, so the water you actually dose is less.
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Real water volume is usually 10 to 20% less than the glass volume after substrate, rock and the gap at the top. Always follow product labels for dosing.
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Good to know
How water changes lower nitrate
Each change removes that share of the nitrate, if your new water has none. A 50% change takes 80 ppm to 40 ppm, and a second takes it to 20 ppm. Test your tap water first: if it contains nitrate, water changes can't go below it.
Doing it safely
Match temperature and dechlorinate. If your tank has gone a while without changes, its pH and hardness may have drifted from your tap's. Test both, and if they differ, make several smaller changes over a few days instead of one big one.