Separate the amount calculation from the decision about the maintenance schedule.
If you choose a 25% water change for 120 litres of actual water, the amount is 30 litres. The arithmetic is exact once the two inputs are chosen.
Stocking, feeding, filtration, plant growth, source water and measured ammonia, nitrite and nitrate all affect maintenance decisions. A calculator cannot infer those conditions from tank litres alone.
Original diagram supporting the how much water should you change? explanation.
A nominal 120-litre aquarium might contain only 105 litres after displacement. Twenty-five per cent of that is 26.25 litres. Be consistent about which volume your routine uses.
Clear water can still contain invisible waste. Use appropriate testing and care guidance rather than treating clarity as proof of good water quality.
Recording test results, water-change amounts and dates makes it easier to see whether a routine is stable than relying on memory or a generic calendar.
Writing “25% / 26 L” in a maintenance note is more useful than writing only the percentage. It preserves the actual action taken and exposes any later change in the water-volume assumption.