Posted by @Chris_Lewis · 1h ago
Salinity is the most important parameter in a marine tank, and the one most often measured wrong. Here is how to get it right.
Why it matters. Corals and invertebrates are sensitive to salinity, and what hurts them most is not the exact number but sudden change. A reef runs best around 1.025–1.026 specific gravity (about 35 ppt); fish-only systems tolerate a touch lower. Consistency beats chasing a perfect value.
Hydrometer vs refractometer. Swing-arm hydrometers are cheap and notoriously inaccurate — bubbles, temperature, and salt creep all throw them off. A refractometer measures how light bends through the water and is far more reliable. Calibrate it with RODI water (should read 1.000) or calibration fluid, and re-check monthly.
Top-off vs water change. Water evaporates but salt does not, so evaporation raises salinity. You replace evaporation with fresh RODI only (an auto top-off makes this painless), and you replace actual water changes with new saltwater mixed to your target. Mixing those two up is a common cause of slow salinity drift.
If you are still scoping a build, the glossary breaks down salinity and specific gravity, and you can sanity-check livestock in the species database.
Your turn: what are you using to measure salinity, and what does your top-off routine look like?