Aquarium salt mix calculator
Find how much marine salt mix to add to reach your target salinity, for brackish and saltwater tanks.
How the amount is worked out
Salinity is often measured as specific gravity, where clean fresh water reads 1.000 and full strength seawater near 35 parts per thousand reads about 1.026. The calculator converts your target specific gravity to parts per thousand, then works out the salt needed from the fact that roughly half a cup of marine mix, about 130 to 140 grams, brings one US gallon up to a reef target. It scales that to your volume and target and shows the result in pounds, grams and approximate cups.
Brands vary a little, so treat the figure as a close starting point rather than an exact dose. Always confirm the final reading with a refractometer or a good hydrometer, since guessing salinity is one of the most common causes of stress in marine and brackish tanks. Mix in a clean bucket or a dedicated mixing station, not in the display tank with livestock present.
Picking a target
Reef tanks with corals usually sit at 1.025 to 1.026, close to natural seawater. Fish only saltwater tanks are often kept a little lower, around 1.020 to 1.023, which is easier on the fish and less costly in salt. Brackish tanks for species like mollies, figure eight puffers or archerfish need only a fraction of that, roughly 1.005 to 1.010, so enter a lower target for those.
Whatever your target, use reverse osmosis or dechlorinated water, add the salt gradually while the water circulates, and let it mix and aerate until it is clear. Bring the new water to the same temperature as the tank before it goes in, and change only part of the volume at a time so salinity stays stable.
Frequently asked questions
How much salt mix do I need per gallon?
For a reef target near 1.025 specific gravity you need roughly half a cup, about 130 to 140 grams, of marine salt mix per US gallon. This tool scales that to your exact volume and target.
What specific gravity should I aim for?
Reef tanks usually target 1.025 to 1.026, fish only marine tanks around 1.020 to 1.023, and brackish tanks much lower at about 1.005 to 1.010. Pick the target that suits your livestock.
Can I use this salt for a freshwater tank?
No. Marine salt mix is for brackish and saltwater tanks. It is not the same as freshwater aquarium salt or tonic salt used to treat freshwater fish, so do not use this tool for those.
How should I mix the salt?
Add salt gradually to reverse osmosis or dechlorinated water, never the other way around, then mix and aerate until it is clear and fully dissolved. Check the final salinity with a refractometer and match the temperature before adding livestock.