Ideal reef tank parameters
| Parameter | Target range | Natural seawater |
|---|---|---|
| Alkalinity | 7–11 dKH (SPS 7–8) | ~6.5–7 dKH |
| Calcium | 400–550 ppm | ~420 ppm |
| Magnesium | 1250–1400 ppm | ~1280 ppm |
| Salinity | 34–36 ppt (1.025–1.027) | 35 ppt |
| Temperature | 76–82 °F (24.4–27.8 °C) | varies |
| pH | 7.8–8.55 | 8.0–8.3 |
| Nitrate (NO₃) | 5–50 ppm | < 0.1 ppm |
| Phosphate (PO₄) | 0.06–0.3 ppm | ~0.005 ppm |
These are the ranges Randy Holmes-Farley recommends. Two things to take from the table before the details: "ideal" is a band, not a single number, and — look at the last column — a healthy reef is not just natural seawater. Recommended alkalinity sits above seawater, and recommended nutrients sit far above it.
Stability beats hitting a number
The most important rule isn't in the table. A tank held steady anywhere inside a range is healthier than one chased back and forth across it. A reef parked at 7.5 dKH is doing better than one bounced between 8 and 10 by an owner chasing "8.5." Pick a target inside the band, then defend stability — don't chase decimals.
The big three: alkalinity, calcium, magnesium
These three do most of the work, and they're linked, not independent.
- Alkalinity — 7–11 dKH. The one that moves fastest and matters most day to day. How to raise it · why it drops.
- Calcium — 400–550 ppm. Consumed alongside alkalinity as coral calcifies (~7.1 ppm Ca per 1 dKH), which is why calcium looks stable while alkalinity moves. How to raise it.
- Magnesium — 1250–1400 ppm. The one people forget. It's what lets the tank hold calcium and alkalinity up at all — correct it first. How to raise it.
Salinity, temperature, pH
- Salinity 34–36 ppt (1.025–1.027). Keep it steady; drift usually means top-off, not salt.
- Temperature 76–82 °F. Stability matters more than the exact figure within the band.
- pH 7.8–8.55. It swings over the day with light and CO₂; the daily average and stability matter more than a single reading. High indoor CO₂ is the usual reason a whole tank runs low.
Nutrients — where "zero" is a mistake
Nitrate and phosphate are the surprise: the recommended ranges are well above natural seawater, and driving them to zero is a risk, not a win — it's the classic setup for pest dinoflagellates, and starves coral of the nutrients it needs (pale colour is a sign). Aim for nitrate 5–50 ppm and phosphate 0.06–0.3 ppm, and never chase both to zero. Why zero nutrients is bad.
If you grow SPS on an ultra-low-nutrient system, tighten alkalinity to 7–8 dKH rather than the full range — these corals get "burnt tips" when alkalinity is too high or moves too much. Here, stability matters more than anywhere else.
Frequently asked
Reef Stability tracks these against the sourced ranges, shows what's in range at a glance, and — the part that matters — tells you when a reading is fine and best left alone. Stability over chasing, built in.
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