Water change calculator
Change 30% of your net water volume each week. For a 52 litre tank that is 16 litres — and the dechlorinator goes into the new water only: about 0.4 ml at 1 ml per 40 litres. Marine tanks additionally need roughly 35 g of salt mix per litre of replacement water.
Not sure of your net volume? Work it out here.
Salt mix applies to marine tanks only — 35 g per litre of replacement water lands near 35 ppt (SG 1.025).
The formula
The rounding is deliberate: you measure whole litres, so the number you act on should be a whole litre. AquaGuru does the same thing in the app and keeps the percentage adjustable, because a heavily stocked cichlid tank and a lightly stocked shrimp tank do not want the same schedule.
Worked example: a 52 litre planted tank
| Change | Calculation | Replace |
|---|---|---|
| 10% | round(52 × 0.10) | 5 L |
| 20% | round(52 × 0.20) | 10 L |
| 30% (default) | round(52 × 0.30) | 16 L |
| 50% | round(52 × 0.50) | 26 L |
At 16 litres, the conditioner dose is 0.4 ml (16 ÷ 40). A reef version of the same change needs 560 g of salt mix (16 × 35) mixed to 35 ppt.
What people get wrong
- Calculating on gross volume. A 60×36×30 cm tank is 64.8 L gross but ~52 L net. 30% of gross is 19 L — a 20% bigger change than intended, every week.
- Dosing conditioner for the whole tank. Conditioner neutralises the water you add, not the water already in the tank. Dosing for total volume wastes product and over-doses.
- Ignoring the bottle. The 1 ml / 40 L rate is a reference. Concentrated conditioners can dose at 1 ml / 200 L, and some are dosed per total tank volume — the label wins.
- Mixing salt by the maths alone. 35 g/L is a starting point. Salt brands differ in moisture and density, so always finish with a refractometer at 35 ppt / SG 1.025.
- Big changes with cold water. A 50% change with water 4 °C colder than the tank is a temperature shock, not a water change. Match temperature before adding.
Questions
How many litres should I change in a 52 litre tank?
About 16 litres at 30% of net volume. Use 10 L for a 20% change, 26 L for 50%.
Conditioner for the whole tank or only the new water?
New water only — 0.4 ml for a 16 L refill at 1 ml per 40 L. Check your bottle for the real rate.
Is a 50% water change too much?
Not if the new water matches temperature and parameters. For emergencies, prefer two or three changes over a few days to a single 80% change.
How much salt does a 16 litre reef change need?
Roughly 560 g at 35 g/L. Mix to your refractometer: aim for 35 ppt or SG 1.025.
Gross or net volume?
Net. Substrate and hardscape leave you with less water than the box says, and the percentage applies to water that exists.
AquaGuru keeps the weekly change on your calendar, calculates the conditioner from the product in your own cabinet, and logs the change when it is done.