Salinity, alkalinity, calcium, nitrate and the rest — what to aim for, and why stability beats a perfect number.
6 questions, answered honestly and water-first.
The reef-standard targets: salinity ~35 ppt (SG 1.026), temperature 75–82°F, pH 8.1–8.4, alkalinity 8–11 dKH, calcium 400–450 ppm, magnesium 1300–1400 ppm, and nitrate and phosphate low but NOT zero. Ammonia and nitrite should read zero. See the full cited chart for every range and what to do when one drifts.
Aim for 35 ppt — specific gravity 1.026 at 77°F, the same as natural seawater. Fish-only tanks can sit a touch lower, but corals and inverts want full reef salinity. Measure with a refractometer (not a swing-arm hydrometer), and remember specific gravity changes with temperature.
A common routine is 10–20% weekly or every couple of weeks, adjusted to your nitrate and phosphate readings. Always mix and age the new saltwater to the right salinity and temperature first — never pour unmixed salt into the tank. Consistent small changes beat occasional big ones.
Only once your corals use those elements faster than water changes can replace them — a lightly-stocked or soft-coral tank often doesn't need dosing at all. Test first; if alkalinity or calcium drifts down between water changes, then dose to hold them stable. Never dose blind.
High nutrients feed nuisance algae, so bring them down with water changes and feeding control. But zero is the opposite problem: corals need some nutrients, and stripping the water bare invites dinoflagellates and pale, starving corals. Aim low-but-not-zero — nitrate roughly 2–15 ppm, phosphate ~0.03–0.10 ppm.
Because corals adapt to a stable value but are hurt by swings. A steady alkalinity of 8.5 dKH beats one bouncing between 7 and 11 chasing "perfect." Pick sensible targets, then change things slowly — small, gradual corrections, never big jumps.
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Short answers from the Coralkin team — kind, general guidance, not a diagnosis. When something looks wrong, test your water first, and for a serious case ask an experienced reefer or an aquatic vet.
