Ideal reef parameters: the sourced version

The eight numbers

Every parameter table you've ever seen converges on the same eight: salinity, alkalinity, calcium, magnesium, pH, nitrate, phosphate, temperature. There's reasonable disagreement at the margins — Randy's "optimal" ranges run a degree or two tighter than Triton's, Red Sea's lab tests for SPS tanks tighter still — but the bands are 90% overlapping. What follows is the consensus midpoint with the primary citation.

Salinity

Target: 35 ppt / 1.0264 specific gravity at 25 °C. This is natural seawater. The Global Seafood Alliance reference and the PSS-78 definition both anchor on this number. Randy's specific gravity article walks through the temperature-correction headache in detail.

Acceptable range: 32–37 ppt, but there's no benefit to either end. Refractometers are the standard hobbyist tool — calibrate with 35 ppt fluid, not RO. Hydrometers drift with temperature and bend the plastic — disposable for a starting tank, replaceable with a refractometer when you have $30. See the salinity reference and salinity calculator.

Alkalinity

Target: 8–9 dKH. Acceptable: 7–11 dKH. Randy's optimal parameters thread defends 8–9 dKH as the sweet spot. Triton runs slightly higher at 8.0–8.5. Red Sea's SPS "low nutrient" protocol runs 7.0–7.5. Forum users will tell you 12 dKH is fine. It isn't — over 10 dKH the precipitation kinetics start working against you and tissue burns appear at coral bases.

The actual rule: pick a set point in the 7.5–9.0 band and hold it within ±0.3. The number matters less than the stability. See alkalinity reference.

Calcium

Target: 420 ppm. Acceptable: 380–450 ppm. Natural seawater is about 412 ppm Ca (University of Washington seawater mineral reference). Randy's consensus is 380–450. Forum lore inflates this to "500 ppm for SPS growth" — no published evidence supports that as beneficial, and it correlates with abiotic CaCO₃ precipitation on heaters and pump impellers.

Move calcium slowly. 20 ppm per day is a safe rate of change. See calcium reference.

Magnesium

Target: 1,350 ppm. Acceptable: 1,250–1,400 ppm. Natural seawater is approximately 1,290 ppm Mg. Randy's magnesium in reef aquaria article defends the slight elevation over natural — Mg substitutes into coral calcite at a rate that hobbyists need to replenish, and the buffering effect at 1,350 ppm keeps Ca and Alk dosing kinetically stable.

Below 1,200 ppm, calcium and alkalinity start precipitating together no matter how much you dose. That's the "my dosing stopped working" symptom — almost always a Mg deficiency. See magnesium reference.

pH

Target: 8.0–8.4 daytime. Acceptable: 7.8–8.5. pH is an output, not a target you dose toward directly. It's set by your alkalinity and the dissolved CO₂ in your tank water, which is downstream of the CO₂ in your room. The UKAPS pH/dKH/CO₂ table lets you read the three off each other.

Trying to dose pH directly with limewater or kalk on a basement tank is a treadmill. The real fix is fresh-air intake to your skimmer — pull room air from outside via a hose, and pH rises 0.2–0.3 within hours. See pH reference and the CO₂ / pH calculator.

Nitrate

Target: 2–10 ppm NO₃. Acceptable: 0.5–25 ppm. Randy's consensus places the target at "measurably above zero, but well below where algae explode." Old-school doctrine was "NO₃ as close to zero as possible." That doctrine is dead — the 2010s ULNS (ultra-low nutrient system) crashes proved it. Coral needs nitrogen.

Sustained zero nitrate plus near-zero phosphate is the dinoflagellate red carpet. If you're reading zero on both, dose nitrate (sodium nitrate or NeoNitro) until you measure 5 ppm. See nitrate reference.

Phosphate

Target: 0.04–0.08 ppm PO₄. Acceptable: 0.02–0.10 ppm. Randy's phosphorus in reefs article covers the chemistry. Natural surface ocean PO₄ is ~0.005 ppm, but reef tanks are not oligotrophic ocean — the consensus reef tank target is 5–15× higher because coral is fed more in captivity than in the wild.

Phosphate inhibits CaCO₃ crystallization at high concentration, which is why "my alkalinity dose stopped working" sometimes traces to PO₄ above 0.15 ppm. Same dynamic as Mg deficiency, different mechanism. See phosphate reference.

Temperature

Target: 78 °F (25.6 °C). Acceptable: 76–81 °F. Tropical reef water in nature runs 77–86 °F seasonally, so we're at the low end of that natural range — by hobby choice, because cooler tanks have higher dissolved oxygen and parasites move slower.

Stability matters far more than the integer. 78 ± 0.5 °F is the goal, achieved with a quality heater and a separate controller (Apex, Neptune, GHL, InkBird). Use two thermometers. Always two thermometers. See temperature reference and the heater wattage calculator.

Stability over set point

If there's one rule that supersedes any specific number above, it's this: coral reads rate of change, not absolute value. A tank at 7.5 dKH for two years grows coral better than a tank that bounces 7–10 dKH every week. Pick numbers within the consensus bands, hold them, and stop adjusting.

The Reef Health Dashboard evaluates a single set of test results against these consensus ranges and flags drift directions. It's not a substitute for keeping a logbook — but if you're starting a logbook today, the dashboard is the right shape.