Keeping saltwater and reef tanks at a stable, correct salinity is one of the most important routine maintenance tasks in marine aquarium keeping. This calculator translates a current-to-target salinity change into a concrete amount of marine salt mix or freshwater to add, so you're not guessing at dosing.

How the Saltwater Aquarium Salinity Calculator works

Enter the dry mix mass, reference US-gallon volume and reference SG from your product label or measured batch. The estimate scales that yield linearly across the SG change and assumes the same volume and temperature basis. No generic 0.5 lb/gal manufacturer rate is used. This is a planning approximation; follow the actual product mixing directions and measure/re-test. Converter paired values clear when their source is empty or invalid.

Use your product’s current label or a measured batch at its stated temperature and volume basis. No universal lb/gal default is supplied. Follow the manufacturer’s mixing instructions and re-test with a calibrated instrument; never mix dry salt in an aquarium containing animals. Red Sea.

Inputs and what they mean

Tank size in gallons should reflect your actual system water volume, including sump and refugium if you have one — not just the display tank. Current specific gravity comes from your hydrometer or refractometer reading; target specific gravity is where you want to end up, commonly 1.025–1.026 for reef tanks or a bit lower (1.020–1.023) for fish-only setups. The mode toggle switches between the two formulas above depending on whether you're raising or lowering salinity.

Limits and edge cases

This calculator gives a starting estimate, not an exact dose — actual marine salt mixes vary somewhat by brand and batch, and hydrometers can drift out of calibration over time, so always re-test with your own instrument before and after adjusting. Change salinity gradually (over hours, not all at once) in an established, stocked tank; sudden swings in either direction stress fish and invertebrates far more than the same total change made slowly. This tool is informational only and is not a substitute for species-specific husbandry research or professional aquaculture guidance.