Reef Tank Water Parameters: Ideal Ranges for Corals
Healthy corals start with stable water. You don't need perfect numbers, but you do need to know your ranges, test regularly, and avoid sudden swings.
Ideal ranges for a mixed reef
| Parameter | Target range | How often to test |
|---|---|---|
| Salinity | 1.025 to 1.026 SG (about 35 ppt) | Weekly |
| Temperature | 76 to 80°F | Daily (check your thermometer) |
| pH | 7.8 to 8.4 | Weekly |
| Alkalinity | 7 to 11 dKH | 1 to 2 times a week |
| Calcium | 400 to 450 ppm | Weekly |
| Magnesium | 1,250 to 1,400 ppm | Every 1 to 2 weeks |
| Nitrate | 2 to 10 ppm | Weekly |
| Phosphate | 0.03 to 0.1 ppm | Weekly |
| Ammonia and nitrite | 0 | When cycling or if something seems wrong |
Soft corals tolerate wider ranges. SPS corals need the tightest control, especially for alkalinity.
Stability beats perfection
A tank that holds steady at 8 dKH is healthier than one that swings between 7 and 10. When a number drifts, correct it slowly over several days. Fast corrections, especially for alkalinity, can stress corals more than the original problem.
Why nutrients shouldn't be zero
Nitrate and phosphate feed algae in excess, but corals need small amounts too. Tanks with zero nutrients often see corals pale, shrink or stay closed. Aim for detectable, low levels.
Keeping parameters stable
- Do regular water changes, such as 10% every week or two.
- Top off evaporation with fresh RO/DI water, not saltwater, to keep salinity steady.
- Dose alkalinity and calcium consistently as your coral load grows.
- Log your test results so you can spot trends early.
Shop our water testing supplies. If corals are closing up, check our troubleshooting guide.
