Reef salinity planning tool
Reef Salinity Adjustment Calculator: Freshwater & Premixed Water Exchange
Estimate the constant-volume water exchange required to move from a measured current salinity toward a user-entered target using freshwater or measured pre-mixed replacement water.
Planning only: this calculator performs transparent mass balance. It does not choose a salinity target, does not define a livestock-safe correction speed, and there is no universal ppt-per-day rule.
Constant-volume mass-balance planner
Estimate a water-exchange volume from measured salinity
Enter your Actual system water volume, measured Current salinity, chosenTarget salinity, and the Replacement-water salinity. All salinity entries are in ppt.
Planning only: this arithmetic does not define a livestock-safe correction speed and there is no universal ppt-per-day rule.
Enter measured values to estimate the constant-volume exchange.
To raise salinity in a stocked aquarium, use pre-mixed saltwater. Do not pour dry salt into a stocked display.
What the calculator solves
At a constant final water volume, replacing a fraction of aquarium water with replacement water produces an idealized mixed salinity:
Sfinal = Scurrent × (1 − f) + Sreplacement × f
Rearranging for the replacement fraction gives f = (Scurrent − Starget) ÷ (Scurrent − Sreplacement). The calculator multiplies that fraction by your actual system water volume and reports both US gallons and liters.
Use actual system water volume
Nominal tank size can differ from real water volume because of rock, sand, operating waterline and sump volume. If you need a transparent estimate, start with the aquarium volume calculator.
Any volume error scales the exchange-volume result directly. The arithmetic cannot compensate for a bad volume assumption.
Lowering salinity
For a verified high-salinity condition at the correct operating water level, freshwater has a replacement-water salinity near 0 ppt. The tool estimates the fraction of saltwater that would be removed and replaced with that freshwater under the constant-volume assumption.
This math is not a recommendation to perform the whole calculated exchange at once. Use the reef salinity correction guide for verification, root-cause diagnosis, conservative adjustment and livestock-safety context.
Raising salinity
Enter the measured salinity of pre-mixed replacement water above your chosen target. The calculator rejects replacement water that cannot mathematically reach the target.
Do not pour dry salt into a stocked display. Prepare and verify replacement water first. The reef saltwater mixing guide covers manufacturer-specific mixing, timing, temperature and aeration.
Why the tool uses ppt inputs
The arithmetic needs values on the same salinity scale. If your instrument displays specific gravity, PSU or another output, first understand the reference model in salinity vs specific gravity. Do not blindly convert a temperature-dependent SG reading into ppt with an unqualified table.
Use a calibrated instrument from the appropriate method. The aquarium salinity tester guide compares conductivity, digital/analog refractive-index and hydrometer workflows.
Assumptions and limitations
This tool assumes accurate current, target and replacement salinity values; complete mixing; a known water volume; a constant final water volume; and no simultaneous evaporation, ATO addition, dosing, leaks or other salt gain/loss during the modeled exchange.
It does not model animal physiology, osmotic acclimation, temperature, oxygenation, species-specific tolerance or how long the aquarium has been outside its established salinity.
Therefore the calculated exchange volume does not define a livestock-safe correction speed. Retest between real adjustments and seek an aquatic veterinarian or experienced reef professional for severe or symptomatic cases.
Methodology boundary
PetGearReport has not tested your aquarium, measured its true water volume, validated your salinity instrument or induced salinity errors in livestock. This calculator is a deterministic mass-balance planner using values you enter.
No Amazon price, availability, affiliate product table or brand-specific correction table is used.