Target salinity range

Standard reef target: 1.025 specific gravity, which equals 35 parts per thousand (ppt) or about 53 milliSiemens per centimeter conductivity. Below 1.022 stresses corals; above 1.028 also stresses corals and pushes calcium and alkalinity chemistry into harder-to-balance territory.

Like every other reef parameter, stability matters more than the exact number. A tank steady at 1.024 is healthier than one bouncing between 1.022 and 1.027 every week.

Why salinity drifts

Water evaporates from a reef tank every day. Salt does not. So as water leaves, the salinity of what remains climbs. A 100-gallon tank can lose 1–2 gallons per day in a dry climate, which raises salinity by 0.005 SG per day. Without automatic top-off, salinity climbs constantly.

When you top off with freshwater, salinity drops back. When you do a water change with newly mixed saltwater, salinity reflects the new mix's strength. So you have three potential drift sources: evaporation, top-off accuracy, and salt-mix consistency.

  • Evaporation only removes water, not salt — raises salinity over time
  • ATO (auto top-off) refills evaporated water with RO/DI — keeps salinity stable
  • Water changes can drift salinity if mixed salt is too strong or too weak

How to test salinity accurately

  • Refractometer calibrated with 35 ppt reference solution — most-recommended single tool, sub-$50. Re-calibrate every 1–3 months; refractometer drift is the #1 source of fake-out salinity readings.
  • Conductivity probe (Apex, HYDROS, Alkatronic) — continuous monitoring, reads in milliSiemens; convert to SG or ppt. Most accurate but needs occasional recalibration.
  • Hydrometer (swing-arm) — cheap, less accurate. Avoid for serious reefkeeping; refractometer is a clear upgrade.
  • Brix-style refractometer that reads "salinity ppt" but is calibrated for fruit juice — common cheap unit; needs the 35 ppt seawater reference solution to read reef tanks correctly.

ATO (auto top-off) is the answer

An automatic top-off system measures evaporation, refills with RO/DI freshwater, and keeps salinity perfectly stable. Cost: $80–$300 depending on quality. Every serious reef tank should have one. The two big risk areas: probe failure (overflow if it sticks "needs water") and reservoir running dry (salinity climbs fast). Most modern ATO units have probe redundancy and reservoir level sensors.