Why does specific gravity depend on temperature?

Water density falls as it warms, so the same salt content reads as a lower specific gravity in warmer water. A reading must be tied to a reference temperature to be meaningful. This converter standardises to specific gravity at 25C referenced to pure water at 25C, the convention most reef hydrometers use.

What salinity should a reef tank be?

Natural seawater is about 35 ppt, which is roughly 1.0264 specific gravity at 25C or about 53 mS/cm conductivity. Most reef keepers target 34 to 36 ppt. Fish-only marine tanks tolerate a slightly wider range, around 30 to 35 ppt.

How accurate are swing-arm hydrometers?

Plastic swing-arm hydrometers are convenient but drift and trap bubbles, often reading several points off. A calibrated refractometer or a conductivity probe is far more reliable. Use this converter to cross-check whichever instrument you own.

What is the difference between ppt and PSU?

Parts per thousand (ppt) is a mass fraction of dissolved salts, while the practical salinity unit (PSU) is defined from conductivity ratios. Near typical seawater values the two are numerically almost identical, so this tool treats them as interchangeable for aquarium use.

Does conductivity change with temperature too?

Yes, conductivity rises with temperature, but most aquarium conductivity meters report a temperature-compensated value referenced to 25C. The conductivity figure here is that 25C-referenced value, so read your meter in its compensated mode.

Aquarium Salinity / Specific Gravity Converter

Marine and reef aquariums live or die by stable salinity, yet hobbyists measure it three different ways: refractometers in ppt, hydrometers in specific gravity, and probes in conductivity. This converter ties all three together and corrects specific gravity for the temperature at which you took the reading.

How it works

Salinity in ppt is the master value. Specific gravity for seawater is estimated from a density relation, and the reading is corrected from your measurement temperature back to the 25C reference using the thermal expansion of water:

density(S, T)  ≈ rho_pure(T) + S × (0.667 + 0.0008 × T) ... g/L scale
SG(25/25)      = density(S, 25C) / density(0, 25C)
conductivity   ≈ S × 1.5 mS/cm  (near seawater, 25C compensated)

The pure-water density curve and the salt-density slope reproduce standard seawater values closely in the aquarium range: 35 ppt gives about 1.0264 specific gravity at 25C and roughly 53 mS/cm.

Tips and targets

Aim for 35 ppt (about 1.0264 at 25C) in a reef tank and keep it stable rather than perfect. Always read a hydrometer at, or corrected to, its calibration temperature; a reading taken in 30C water will under-report specific gravity if you assume 25C. Calibrate refractometers with a known standard solution monthly, and rinse swing-arm hydrometers to clear bubbles before trusting them.