Reef Lab
Salt mix calculator
How much salt for the batch you are mixing — and, if the tank has drifted, how much salt or RO water it takes to walk the salinity back without shocking anything.
Mixing new saltwater
Four steps, and the order matters more than people expect.
Start with RO or RO/DI water. Tap water brings nitrate, phosphate, silicate and copper along. Copper in particular is a slow, invisible killer for inverts, and no salt mix corrects for it.
Salt into water, never water onto salt. Add the mix slowly to moving water with a powerhead running. Dumping water onto a pile of salt precipitates calcium carbonate — the batch goes cloudy and permanently loses some of its alkalinity and calcium.
Heat it, then wait. Bring the batch to tank temperature and let it circulate. A few hours is the minimum, overnight is the norm. Water that is still cloudy is not ready.
Measure before you use it. Check salinity in the finished, temperature-matched batch, not while it is still warming up. That is the only reading that means anything.
Never pour dry salt into the display
Salt dissolving in the tank creates a local pH and alkalinity spike, and grains that settle on a coral burn it chemically. The single exception is a small top-up: dissolve it in a cup of tank water first and add it to the sump, slowly.
Moving a tank that has drifted
Up is easy. Down needs water, not wishful thinking.
To raise salinity you add salt, and the amount scales straight with volume. To lower it there is no trick available: the salt already dissolved is not going anywhere, so you add RO water and spread the same salt over more of it.
V × Scurrent = (V + x) × Starget
The dissolved salt mass stays constant. A 300 l tank at 36 ppt needs 8.6 l of RO water to land on
35 ppt — which is also why an evaporation top-off has such a direct effect on your salinity.
Whichever direction you go, keep it under roughly 1 ppt a day. The calculator splits a larger jump across as many days as the salt's own safe daily limit requires, and tells you the per-day amount.
| Salt mix | Per litre | Per US gallon | Maker's own figure |
|---|---|---|---|
| Red Sea Salt | 38.2 g | 145 g · 5.10 oz | 38.2 g/l → 35 ppt |
| Red Sea Coral Pro | 40.6 g | 154 g · 5.42 oz | 40.6 g/l → 35 ppt |
| Tropic Marin Pro-Reef | 37.5 g | 142 g · 5.01 oz | 37.5 g/l → 35 ppt |
| Tropic Marin Classic | 38.5 g | 146 g · 5.14 oz | 37–40 g/l → 32–35 ppt |
| Fauna Marin Professional | 39.0 g | 148 g · 5.21 oz | 38.5–39.5 g/l → 35 ppt |
| Aquaforest Reef Salt | 41.4 g | 157 g · 5.52 oz | 39 g/l → 33 ppt |
| Aquaforest Sea Salt | 40.3 g | 152 g · 5.37 oz | 34.5 g/l → 30 ppt |
| Brightwell NeoMarine | 37.3 g | 141 g · 4.98 oz | 35.4 g/l → 1.025 |
| Instant Ocean Sea Salt | 43.0 g | 163 g · 5.75 oz | 35.9 g/l → 1.022 |
| Instant Ocean Reef Crystals | 42.2 g | 160 g · 5.63 oz | 33.6 g/l → 1.021 |
| Seachem Reef Salt | 42.3 g | 160 g · 5.65 oz | 33.8 g/l → 1.021 |
Read the last column before you trust the first. Only some makers state a figure for 35 ppt; the others publish theirs at a much lower salinity — Instant Ocean at 1.022, Seachem at 1.021 — and the 35 ppt column extrapolates from there in proportion. That extrapolation is sound arithmetic but a long way from the anchor, which is exactly why every batch still gets measured before it goes in the tank.
Questions
How much salt do I need per litre or gallon?
Somewhere between 37 and 43 grams per litre, or roughly 5 to 5.8 ounces per US gallon, depending on the mix. Red Sea Salt states 38.2 g/l for 35 ppt and Tropic Marin Pro-Reef 37.5 g/l, while Aquaforest Reef Salt works out at about 41.4 g/l because its own figure is quoted at 33 ppt. The spread between brands is real and worth looking up once for the salt you buy — and the bucket's own instructions always outrank any table.
Can I add dry salt straight to my aquarium?
No. Dry salt mix dissolving in the display creates a locally extreme pH and alkalinity spike, and undissolved grains land on corals as a chemical burn. Always dissolve salt in a separate container of RO water first, let it mix and reach tank temperature, and only then add it. The one exception is topping up salinity, where a small amount of salt is dissolved in a cup of tank water and added slowly to the sump.
How long should new saltwater mix before I use it?
Give it a powerhead and at least a few hours; overnight is the common practice and 24 hours is plenty. The salt needs time to dissolve fully and the water needs to come to tank temperature. Water that looks cloudy is not finished. Some salts also settle their alkalinity in the first hours, so a batch tested straight after mixing can read differently from the same batch the next morning.
How do I lower the salinity in my tank?
Only with fresh RO water — there is no way to remove salt from water you keep. The dissolved salt stays put and simply spreads over a larger volume, so the amount needed follows from V multiplied by current salinity equals V plus x multiplied by target salinity. In practice you do it in steps over several days, or simply do water changes with slightly lower-salinity water. Never drop more than about 1 ppt per day.
Should new water match my tank's salinity exactly?
For a routine water change, yes — match the tank so the change is invisible to the animals. Mixing to a different salinity is how you deliberately move the tank, and then the difference should be small enough that even a large change shifts the display by well under 1 ppt. Remember that evaporation between changes raises salinity on its own, which is what an auto top-off is there to cancel out.
The other calculators
Same maths as the app, same free access.
Salinity calculator
Specific gravity to ppt with the temperature correction most converters skip.
Dosing calculator
Raise alkalinity, calcium or magnesium to a target, split over as many days as it takes.
Consumption calculator
Two readings, a few days apart — out comes the daily dose your pump should run.
Your water changes, on record
Reef Lab logs every change with volume, salt used and date, and keeps the salinity chart next to it — so you can see what the tank actually did. Free, works offline, no account needed to start.