What Salinity Should My Reef Tank Be?
Quick Answer
Reef tanks should maintain salinity between 1.024-1.026 specific gravity (or 32-35 ppt). Stability matters more than hitting an exact number — avoid rapid fluctuations.
The ideal salinity for a reef tank is 1.024-1.026 specific gravity (or 32-35 parts per thousand). This range mimics natural ocean conditions and keeps both fish and corals healthy. Most reefers target 1.025 as a middle ground.
Understanding Salinity Measurements
You'll see salinity expressed in different units:
- Specific Gravity (SG): 1.024-1.026 — Most common on refractometers and hydrometers
- Parts Per Thousand (ppt): 32-35 ppt — Used by digital salinity monitors
- Salinity (PSU): 32-35 — Essentially the same as ppt for practical purposes
Why Salinity Matters
Salinity directly affects your tank inhabitants:
- Fish: Regulate their internal salt balance through osmoregulation. Wrong salinity stresses their kidneys
- Corals: Depend on stable salinity for cellular function and growth
- Invertebrates: Particularly sensitive to salinity swings — shrimp and snails may die from rapid changes
Stability Over Perfection
Consistency matters more than hitting an exact number. A tank stable at 1.024 is healthier than one that swings between 1.023 and 1.026. Aim for no more than 0.001 SG change per day.
What Causes Salinity Swings?
- Evaporation: Water evaporates, salt stays behind, salinity rises
- Water changes: New saltwater mixed at wrong concentration
- Top-off with saltwater: Common mistake — always top off evaporation with freshwater
- Equipment issues: Failed ATO (auto top-off) systems
How to Measure Salinity
Recommended Tools
- Refractometer: Most accurate and affordable option. Calibrate with calibration fluid before each use.
- Digital salinity monitor: Convenient but requires probe maintenance and periodic calibration
- Hydrometer: Cheap but less accurate — good for rough checks only
Testing Best Practices
- Test water at room temperature (68-77°F) for accurate readings
- Calibrate your refractometer with 35ppt calibration fluid
- Test weekly at minimum, or after any significant top-offs or water changes
- Record results in the ReefBay app to track trends over time
Mixing Saltwater Correctly
When preparing water for changes:
- Use RO/DI water (not tap water)
- Add salt gradually, mixing thoroughly
- Let it mix for at least 4-24 hours with a powerhead
- Verify salinity matches your tank before adding
- Match temperature to your tank (±2°F)
Common Salinity Mistakes
Topping Off with Saltwater
When water evaporates, only pure water leaves — the salt stays behind. Always replace evaporation with freshwater (RO/DI), never saltwater. An auto top-off (ATO) system automates this.
Not Calibrating Equipment
Refractometers drift over time. Calibrate with proper calibration fluid (not distilled water) before important measurements.
Large Water Changes with Mismatched Salinity
If your new saltwater is 1.028 and your tank is 1.024, a large water change can shock sensitive invertebrates. Always match new water to current tank salinity.
Adjusting Salinity Safely
If your salinity is off, correct it gradually:
- Too high: Top off with extra RO/DI water over several days
- Too low: Add small amounts of saltwater during water changes
- Target rate: No more than 0.001-0.002 SG change per day
Patience prevents stress and keeps your reef healthy long-term.
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