Salinity correction calculator
Salinity too high or too low? Work out how much water to take out and replace, and spread it over days so your livestock adjust.
Formulas and sources last checked . How we check
Calculator
Result
Swap in total, over 2 days11.0 litres
- Replace with
- fresh RO/DI water
- Change per day
- no more than 1.0 ppt
- Day 1: take out 5.41 litres, add the same of RO/DI. 37.0 ppt to 36.0 ppt.
- Day 2: take out 5.56 litres, add the same of RO/DI. 36.0 ppt to 35.0 ppt.
How does the salinity calculator work?
When you mix two waters, the salinity of the result is the average of the two, weighted by volume. To lower salinity you take out tank water and add the same volume of RO/DI water (salinity zero). To raise it you replace tank water with stronger saltwater. The calculator splits the change into equal steps, no larger than the daily limit you set, and works out each day’s swap from where the previous day ended.
to lower: litres to swap = volume x (1 - target / now)to raise: litres to swap = volume x (target - now) / (new saltwater - now)mixed salinity = (V1 x S1 + V2 x S2) / (V1 + V2)
Worked example: 200 litres at 37 ppt
- The tank holds 200 litres of water and reads 37 ppt. The target is 35 ppt, at 1 ppt a day: two days.
- Day 1, 37 to 36: 200 x (1 - 36 / 37) = 5.4 litres out, 5.4 litres of RO/DI in.
- Day 2, 36 to 35: 200 x (1 - 35 / 36) = 5.6 litres out, 5.6 litres of RO/DI in.
- Check with the refractometer each day before the next swap. As a check: 36.00 ppt after day 1.
Why does salinity drift?
Rising salinity usually means the auto top-off has stopped or its reservoir is empty: water evaporates, salt stays. Falling salinity with the water level near the rim usually means the top-off is stuck on. Fix the cause before you correct. Red Sea’s Foundation manual notes that a 1 ppt rise from evaporation also lifts calcium by about 13 ppm and magnesium by about 40 ppm, so correct salinity before you test anything else.
What are the limits?
Other calculators
- Salt mixGrams of salt for a batch, and what your refractometer or hydrometer should read.
- Water change effectWhat a change, or a run of changes, does to a reading.
- ATO reservoir safetyHow far salinity falls if a stuck pump empties the reservoir.
- Dosing from a test resultMillilitres to bring alkalinity, calcium or magnesium back to target.
- Daily consumptionHow much your tank uses a day, and the daily dose to match.
- Unit converterdKH, meq/L, ppm, SG, ppt, litres and gallons, Celsius and Fahrenheit.
- Tank volumeLitres from the tank size, and the net water after rock, sand and sump.
- Heater sizeWatts for a UK room, split across heaters so one stuck on is not a disaster.
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- Electricity running costWhat the tank costs to run at the current UK price cap.
- Stocking and compatibilityCheck a list of fish against each other, the tank and your corals.