Saltwater Aquarium Water Parameters: The Numbers That Keep Coral Alive

Reef tanks fail on chemistry. Not equipment failure, not bad livestock — chemistry that drifted while the aquarist was not testing. This is a reference for the parameters that matter, what range to hold them in, and how often to check.

Salinity and specific gravity

Target: 1.025–1.026 SG (35–36 ppt)

Evaporation raises salinity daily. An auto-top-off (ATO) system that replaces evaporated water with RO/DI freshwater keeps salinity stable. Without one, top off manually every day. A refractometer calibrated with reference fluid is the minimum testing tool; a Milwaukee or similar digital refractometer is more accurate.

Never use uncalibrated swing-arm hydrometers — they read 1–2 ppt low and give false confidence.

Temperature

Target: 77–80°F (25–27°C)

Coral bleaches above 84°F. Stay consistent — swings of more than 2°F in 24 hours stress livestock more than a steady value slightly outside the target range. In summer, a small fan over the sump reduces temperature at low cost before you reach for a chiller.

pH

Target: 8.1–8.4, ideally peaking at 8.3–8.4 during the photoperiod

pH follows a diel cycle — it drops overnight as CO2 from respiration accumulates and rises during the day as photosynthesis consumes CO2. A pH below 7.9 stresses coral. Running the lights on a reverse photoperiod refugium (algae lit when display is dark) buffers the overnight pH drop without dosing.

Test pH with a calibrated probe, not strips. Calibrate two-point with 7.0 and 10.0 buffer at least monthly.

Alkalinity (dKH)

Target: 8.0–10.0 dKH (1 dKH = 17.8 mg/L)

Alkalinity is consumed fastest in a coral-heavy tank — SPS tanks can drop 0.5–1.0 dKH per day. Test every one to two days when keeping SPS, twice weekly for softs and LPS. Never raise alkalinity by more than 0.5 dKH per day; fast shifts cause coral tissue recession that looks like a disease but is a chemistry injury.

Two-part dosing (part A = calcium chloride, part B = sodium bicarbonate) is the most controllable method for maintaining alkalinity and calcium together. A calcium reactor becomes worth it once tank demand exceeds what two-part can meet affordably.

Calcium

Target: 400–450 ppm

Calcium and alkalinity are linked — you cannot raise one without affecting the other if you do not also dose the complement. Use a reference test kit (Salifert, Hanna) not generic aquarium strips. Test weekly for most tanks.

Magnesium

Target: 1250–1350 ppm

Magnesium stabilizes the relationship between calcium and alkalinity. Low magnesium makes it impossible to maintain calcium at target even with dosing, and causes alkalinity precipitation (white precipitate in the sump). Test monthly unless you see unexplained calcium or alkalinity drift.

Nitrate

Target: 1–10 ppm for SPS; up to 20 ppm for mixed reef

Zero nitrate sounds ideal but is not — completely nutrient-depleted tanks cause coral to expel their zooxanthellae (bleach from starvation, not heat). A small nitrate reading means your bioload and filtration are in approximate balance. Test weekly.

Phosphate

Target: 0.03–0.10 ppm

High phosphate (above 0.1 ppm) suppresses coral calcification and feeds algae blooms. Test with a Hanna Phosphate Ultra Low checker — standard test kits are not sensitive enough at reef levels. Phosphate export: protein skimmer, refugium with macroalgae, or GFO reactor.

How to test reliably

Use the same test kit brand across parameters where possible — switching brands can produce readings that look like parameter swings but are just kit variance. Log every test in a spreadsheet with the date. Trends matter more than individual readings; a calcium that has dropped 20 ppm over three weeks tells you more than a single low reading.

Reference standard kits for critical parameters: ICP-OES lab testing (Triton, ATI) every three to six months gives you a full element panel and catches parameter drift that hobby kits miss.