Reef tanks thrive on stable water. Common targets are salinity of about 1.025 to 1.026 specific gravity (about 35 ppt), temperature 76 to 80°F, alkalinity about 7 to 11 dKH, calcium 400 to 450 ppm, magnesium 1,250 to 1,400 ppm, with low but measurable nitrate and phosphate. Stability matters more than chasing exact numbers, so test regularly and change parameters slowly.
Corals respond to swings in water chemistry more than to slightly off numbers. Test, record and change slowly.
Common target ranges
| Parameter | Typical target |
|---|---|
| Salinity | 1.025 to 1.026 SG (about 35 ppt) |
| Temperature | 76 to 80°F |
| Alkalinity | About 7 to 11 dKH |
| Calcium | About 400 to 450 ppm |
| Magnesium | About 1,250 to 1,400 ppm |
| Nitrate | Low but measurable, often about 2 to 10 ppm |
| Phosphate | Low but measurable, often about 0.03 to 0.1 ppm |
| pH | About 7.8 to 8.4 |
Ranges vary between experienced reefers; follow a consistent method.
Testing tools
| Tool | Use |
|---|---|
| Refractometer, calibrated | Salinity |
| Liquid or digital test kits | Alkalinity, calcium, magnesium, nitrate, phosphate |
| Lab tests | Periodic trace element checks |
Keeping it stable
- Automatic top-off replaces evaporated fresh water and holds salinity steady.
- Dosing pumps add alkalinity and calcium in small, regular amounts.
- Protein skimmers remove organic waste before it breaks down.
- Change one variable at a time.
Related: Aquascaping Basics.
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Common questions
What salinity should a reef tank be?
About 1.025 to 1.026 specific gravity, roughly 35 ppt, measured with a calibrated refractometer.
What is a good alkalinity for a reef tank?
Often 7 to 11 dKH. Pick a target and keep it stable.
How often should I test reef parameters?
Alkalinity often several times a week in growing tanks, calcium and magnesium weekly, nutrients weekly.
Should nitrate and phosphate be zero?
Not usually. Many reefers keep low but measurable levels, because corals need some nutrients.