Reef Tank Water Chemistry Explained

🐠 The short version: reef chemistry is just a handful of numbers kept stable. Salinity, temperature and alkalinity are the big three; calcium and magnesium join them for stony corals; nitrate and phosphate stay low but not zero; ammonia and nitrite stay at zero. The golden rule across all of it: a steady number beats a perfect one that swings.

🔧 See every target with its cited source on the reef parameter chart, and size any additions with the dosing calculator.

The numbers, and what each one does

You don’t need a chemistry degree — you need to know what to aim for and why. Here’s the working set (full sourced ranges on the parameter chart):

• Salinity — about 35 ppt (SG ≈ 1.026). The concentration of salt. Measure it with a refractometer, and top up evaporation with fresh RO/DI (never saltwater), because salinity rises as water evaporates. See the salinity converter.
• Temperature — a stable 76–82 °F. Steadiness matters more than the exact figure.
• pH — 8.1–8.4. Usually looks after itself in a well-run tank; chase stability, not decimals.
• Alkalinity — 8–11 dKH (SPS tanks often 7–8), kept stable. The carbonate buffer corals build skeleton from, and the parameter they’re most sensitive to. Swings here cause tissue burn and recession.
• Calcium — 400–450 ppm and Magnesium — 1300–1400 ppm. The other two stony-coral building blocks; magnesium keeps calcium and alkalinity in balance.
• Nitrate — 2–15 ppm and Phosphate — 0.03–0.10 ppm. Coral food. Keep them low but present — see should reef nitrate and phosphate be zero?.
• Ammonia and nitrite — zero. The only readings that should be undetectable; anything above means an uncycled or overloaded tank.

Stability beats the perfect number

If you take one thing from this guide, take this: corals adapt to a steady value far better than they tolerate a swinging one. A tank held rock-steady a little outside the “ideal” band will usually look healthier than one bouncing around inside it. This is truest for alkalinity — a fast alk swing is one of the most common causes of coral browning, recession and tissue loss. So pick sensible targets, then defend consistency: steady top-off, regular water changes, and dosing that keeps up with demand rather than lurching.

When you have to dose

In a lightly-stocked or soft-coral tank, regular water changes usually replace the alkalinity, calcium and magnesium your corals use. As stony corals grow, their demand outpaces water changes and you start dosing to hold the numbers steady. Test to find how fast your tank consumes each, then use the dosing calculator to size the dose — it’s stoichiometry, not guesswork. Two rules: don’t dose calcium and alkalinity at the same point (they precipitate), and raise slowly within safe daily limits.

A sane testing routine

• Fish-only / soft corals: salinity plus the basics, weekly, is plenty.
• Stony corals: track alkalinity, calcium and magnesium often enough to know your tank is stable and to size dosing — weekly at first.
• Always: confirm salinity and temperature are steady before you go hunting for exotic causes of a problem. Most “mystery” reef issues are really water issues.

Sources & further reading

• Coralkin — Reef parameter target chart — every range above, each anchored to a cited authority.
• Coralkin — Dosing calculator — alkalinity, calcium and magnesium dosing, stoichiometry-derived.
• Randy Holmes-Farley — reef water chemistry (via the parameter chart’s citations) — the basis of the salinity and dosing figures.
• Bulk Reef Supply — reef chemistry and nutrient management.