| What It Is | Salt concentration of your water |
| Reef Target | 1.025–1.026 SG (~35 ppt) |
| Best Tool | Refractometer (calibrated) |
| Fish-Only OK | 1.020–1.025 SG |
| Golden Rule | Keep it stable; top off evaporation |
Overview
Salinity is how much salt is dissolved in your water, and for a reef tank it needs to sit in a narrow range and stay there. Corals and invertebrates are far less tolerant of salinity swings than fish. The good news: salinity is easy to measure and control once you understand that evaporation is what moves it — and that you replace evaporation with fresh water, not saltwater.
Target and Measurement
Keep a reef at 1.025–1.026 specific gravity (about 35 ppt), the same as natural seawater. Measure with a refractometer — cheap swing-arm hydrometers are notoriously inaccurate. Calibrate the refractometer with calibration fluid (not just RODI) and rinse it between uses. Fish-only tanks can run a bit lower (1.020–1.025), sometimes intentionally to reduce parasites.
How to Keep It Stable
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- When water evaporates, salt stays behind and salinity rises — so top off evaporation with fresh RODI water, never saltwater.
- Automate top-off if you can (an ATO); steady daily top-off keeps salinity from drifting between water changes.
- Match new saltwater to tank salinity during water changes — mix, measure, and adjust before it goes in.
- Check salinity weekly, and any time evaporation has been unusually high (hot weather, open lids).
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Common Mistakes
Topping off with saltwater: drives salinity ever higher — evaporation loses water, not salt. Trusting an uncalibrated tool: a refractometer off by a few points can push corals out of range without you knowing. Big fast corrections: if salinity has drifted, walk it back slowly over a day or two — sudden swings stress everything.