Aquarium Salt Salinity Calculator
Calculate exactly how much salt to add to your freshwater or marine tank.
Total water volume
Parts Per Thousand (ocean = 35)
What is the ideal salinity for my aquarium?
Freshwater community: 0-1 PPT. Brackish: 3-15 PPT depending on species. Saltwater: 32-35 PPT (typical 33-35). reef: 34-35 PPT. Open ocean is ~35 PPT. Freshwater fish die above 5 PPT, marine life below 30 PPT. Many tolerates range slightly outside their ideal. Stability matters - sudden changes stress fish more than being at the edge of tolerance. Float salinity slowly when adjusting, never exceed 2 PPT change per day.
How much salt should I add?
Add 1 cup (about 27 ounces) of marine salt to each gallon of freshwater for ~35 PPT. Use slightly less (~3/4 cup per gallon) for 25 PPT. Scale to actual tank volume (after subtracting rocks, subtract 10% from display volume). Mix into removed tank water at room temperature, pour back slowly. Refractometers read actual salinity, use test kit to calibrate. Mix min 24 hours before adding fish if adjusting.
How do I measure salinity without expensive equipment?
Refractometers (~$40-60) most accurate for saltwater, read PPT. Hydrometers cheaper ($10-25) but drifting, calibrate with known solution. For freshwater, use specific gravity readings (1.000-1.025 typical). Temperature affects readings - calibrate to 77F/25C reference point if using refractometer. Drop-style salinity testers easier but less precise than digital.
How often should I check salinity?
Top off evaporated water with freshwater only (not salt water) - evaporation removes pure water, minerals stay behind so salinity rises. Check weekly in established tanks, any time after adding water (including during water changes). Check more frequently in new tanks until cycled. Salt creeks in summer and winter when heaters add evaporation: check twice weekly during temperature changes, every few days in heat waves when evaporation increases significantly.