Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Salt mass to bring RO/DI up to a target salinity, and what the mixed water should test at.
| Volume | Grams | Pounds | Cups |
|---|---|---|---|
| 5 gal | 685 | 1.51 | 2.4 |
| 10 gal | 1,369 | 3.02 | 4.7 |
| 25 gal | 3,423 | 7.55 | 11.8 |
| 50 gal | 6,846 | 15.09 | 23.6 |
Mixes 95 L of RO/DI to 35.0 ppt with Generic, physical ideal. Add salt to water, never the reverse. Run a powerhead and a heater for at least six hours, then test before it goes anywhere near the tank.
grams = 94.6 L × 35.0 ppt × 1.0334 = 3,423 g
Salinity in ppt means grams of salt per kilogram of seawater, not per litre of the fresh water you started with. Those differ, and the difference is where most salt mix tables go wrong.
Working it through: seawater at 35 ppt and 25 °C has a density of 1023.34 g/L, so one litre of it holds 35.82 g of salt and 987.5 g of water. That water occupies 990.4 mL before the salt goes in. So a litre of RO water needs 36.2 g of pure sea salt, which is 1.0334 g per litre per ppt.
Commercial mixes are not pure salt. They carry water of hydration and some non-ionic content, so a brand figure sits slightly above the ideal. Where a manufacturer publishes a yield it is used here. Where none exists, the physical ideal is used and the result is flagged.
Manufacturer instructions almost always target 1.021 to 1.023 specific gravity, which is around 30 ppt, well below what a reef tank runs. Half a cup per gallon is a fish store figure, not a reef figure. Expect to use more.
Bucket scoops drift. Salt takes on moisture from the air, and a cup of salt from a bucket opened six months ago weighs less salt and more water than a fresh one. Weighing beats scooping whenever the batch matters.
Mix for at least six hours with a powerhead and a heater, longer for the high alkalinity reef formulas, which need time to fully dissolve. Cloudy water after mixing usually means precipitation from mixing too fast, too warm, or into water that already had salt in it.
Always test the finished batch. Salt mixes vary between batches, refractometers drift, and it costs nothing to check before it goes into the tank.
Sources: Red Sea Salt manual (PDF) · Fritz RPM Reef Pro Mix · Tropic Marin Pro-Reef · Aquaforest Reef Salt · Instant Ocean Sea Salt