Why it matters
Wrong salinity stresses livestock and breaks osmoregulation. Coral and inverts are far less tolerant than fish.
Ideal ranges
| Water type | Ideal | Acceptable |
|---|---|---|
| Reef | 1.025 SG / 35 ppt | — |
| Fish-only saltwater | 1.020 to 1.025 SG | — |
| Brackish (low) | 1.005 to 1.010 SG | — |
| Brackish (high) | 1.010 to 1.018 SG | — |
How to test
- 1
Refractometer (recommended)
Calibrate weekly with 35 ppt calibration fluid, not RO water.
- 2
Conductivity probe
Most accurate. Tunze, Apex, Neptune all sell reliable probes.
- 3
Avoid swing-arm hydrometers
Off by 0.002 to 0.005 SG and crystallise inaccurate over time.
How to correct low salinity
Top off with new salt mix
Mix saltwater to target SG, add slowly over hours.
Check ATO is not adding too much fresh
Auto top-off should add fresh RO/DI only, not saltwater.
How to correct high salinity
Top off with RO/DI fresh water
Add slowly. Drop SG by no more than 0.001 per day.
Verify ATO is functioning
Stuck float switches let salinity creep up via evaporation.
Common causes
- •Evaporation raising SG
- •Top-off failure
- •Mixing salt at wrong ratio
- •Adding livestock without acclimation
FAQs
Why are reef tanks higher SG than fish-only?
Natural seawater is 1.025. Reef coral and inverts evolved at that exact salinity. Fish-only can tolerate slightly lower for cost savings on salt.