A reef tank should sit at about 35 ppt salinity, which reads as a specific gravity of roughly 1.026 at 77°F. That’s natural seawater. Anywhere from 34 to 36 ppt (about 1.025 to 1.027) is fine for corals and invertebrates, and keeping salinity steady matters more than hitting 35.0 exactly.
Why is 35 ppt the reef target? #
Corals and reef invertebrates evolved in open ocean water, which NOAA puts at an average of about 35 parts per thousand, meaning roughly 35 grams of dissolved salts in each kilogram of seawater. The open ocean varies from place to place, but on a given reef it changes very little from day to day, and reef animals handle swings poorly. Fish are more tolerant, which is why fish-only tanks can run lower.
Alkalinity, calcium and magnesium also rise and fall with salinity. If salinity drifts from 35 to 33 ppt, all three drop by about 6% even though nothing consumed them. Get salinity steady first and your other readings become easier to trust.
Salinity vs specific gravity: what’s the difference? #
Salinity, in ppt or the equivalent PSU, is how much salt is dissolved. Specific gravity (SG) is how dense the water is compared with pure water. Density depends on temperature as well as salt, so an SG reading only converts to salinity at a known temperature, and most hobby tables use 77°F (25°C). Refractometers with automatic temperature compensation correct for temperature within their stated range.
Salinity to specific gravity chart #
Approximate values at 77°F (25°C):
| Salinity (ppt) | Specific gravity | Suits |
|---|---|---|
| 30 | 1.0226 | Too low for a reef |
| 31 | 1.0233 | Fish-only |
| 32 | 1.0241 | Fish-only |
| 33 | 1.0249 | Low end for fish-only and some reefs |
| 34 | 1.0256 | Reef, low end |
| 35 | 1.0264 | Reef target, natural seawater |
| 36 | 1.0271 | Reef, high end |
| 37 | 1.0279 | Too high for most reefs |
A handy rule of thumb: near reef salinity, each 0.001 of specific gravity is about 1.3 ppt.
What salinity for fish-only, reef and brackish tanks? #
| Tank type | Typical salinity | Notes |
|---|---|---|
| Reef | 35 ppt (SG about 1.026) | Corals and inverts need full seawater and stability |
| FOWLR or fish-only | Commonly SG 1.020-1.025 | Fish tolerate lower, but any inverts in the tank need more |
| Brackish | Often SG 1.005-1.015 | Varies by species, and some fish move toward full marine as adults |
| Hyposalinity treatment | Around SG 1.009, in a hospital tank | A fish-only parasite treatment that kills corals and inverts |
If a fish-only tank has cleaner shrimp, snails or crabs, treat it like a reef for salinity. We cover the fish-only style in what is a FOWLR tank.
How do you measure salinity accurately? #
- Refractometer. The most common reef tool. Calibrate it with 35 ppt calibration fluid, not just RO water. RO water only checks zero, and a refractometer can be right at zero and still read off at seawater strength. Rinse the prism with RO water after each use, because dried salt crust skews the next reading.
- Digital salinity meter. Reads conductivity and converts it to salinity. Easy to read, but it needs calibrating with the fluid its maker specifies, and probes drift over time.
- Swing-arm hydrometer. Cheap and fine for fish-only tanks, but air bubbles on the arm and temperature both throw it off. It isn’t precise enough for a reef.
Whatever you use, measure at your tank’s normal temperature and write the number down along with the tool you used.
Why does reef tank salinity drift? #
- Evaporation. Water evaporates and the salt stays, so salinity climbs every day until you top off. A small nano can lose enough water in a couple of days to shift salinity noticeably.
- Topping off with saltwater. Top-off water must be fresh RO/DI water. Replacing evaporated water with saltwater raises salinity every time you do it.
- Mismatched water changes. Measure each batch before it goes in. Our salt per gallon guide covers mixing.
- Salt creep and skimmer overflow. Both carry salt out of the tank and slowly lower salinity.
- Auto top-off failure. A stuck valve can drop salinity fast by overfilling with fresh water, and a dead pump lets it climb.
An auto top-off (ATO) system is the biggest single fix for salinity swings. A reservoir of RO/DI water feeds the tank as it evaporates, so salinity stays flat between water changes. Keep that reservoir water pure: if your RO/DI output starts showing TDS, replace the cartridges before that water goes in the tank, as covered in how often to replace RO/DI filters.
How fast can you correct salinity? #
Slowly. A common reef-keeping rule is to change salinity by no more than about 1 ppt (roughly 0.001 SG) per day. To lower it, replace a little tank water with RO/DI water each day. To raise it, do water changes with slightly saltier water, or top off with a small amount of saltwater, and remeasure the next day. Fish-only tanks tolerate faster corrections than corals and invertebrates do.
How do you track salinity over time? #
A salinity chart shows problems that single readings hide: a slow upward creep that means the ATO isn’t keeping up, or a sawtooth that lines up with water changes.
Fish Tank Tracker logs salinity in ppt and charts it against a 34 to 36 ppt band, next to alkalinity, calcium and magnesium so you can see them move together. If your refractometer reads SG, convert with the chart above before you log it. Quick mode takes a few seconds for a salinity check, and the app works offline with no account.
Frequently asked questions #
Is 1.025 okay for a reef tank? #
Yes. 1.025 is about 33 ppt, slightly under natural seawater, and plenty of reefs run there without trouble. Keeping it steady matters more. If you decide to move up to 1.026, do it gradually over several days.
Is 1.028 too high for a reef tank? #
At about 37 ppt, it’s above what most reef keepers aim for. Bring it down slowly with fresh RO/DI water over several days, and check that your top-off reservoir doesn’t contain saltwater.
Does temperature affect salinity readings? #
Temperature doesn’t change how much salt is in the water, but it changes density, so it affects specific gravity readings from hydrometers. Refractometers with automatic temperature compensation correct for it within their range. Measure at your normal tank temperature either way.
How often should I check salinity in a reef tank? #
A few times a week is common, and daily if you don’t have an auto top-off. Also measure every new batch of saltwater before a water change.