Parameter References
Reef parameters
What every number means, where it should sit, what happens when it drifts, and which test kit to actually buy. All ranges cite Randy Holmes-Farley's optimal-parameters consensus.
Alkalinity
7–11 dKHCarbonate buffer. The single most volatile reef parameter — coral reads rate of change, not absolute value.
Calcium
380–450 ppmSkeleton building block. Stable trumps high — pair every adjustment with alkalinity.
Magnesium
1,250–1,400 ppmThe reason Ca and Alk can coexist. Without it, both precipitate together.
Salinity
35 ppt / 1.0264 SG35 ppt natural seawater. Refractometer, not hydrometer.
pH
7.9–8.4An output, not a target. Driven by alkalinity and dissolved CO₂.
Nitrate
2–10 ppmEnd-product of the nitrogen cycle. Not zero — measurably low.
Phosphate
0.02–0.10 ppmCoral nutrient and algae fuel. Same molecule, two outcomes.
Temperature
76–81 °FStability over set point. 78 ±0.5 beats 76–80 swings every time.
Frequently asked questions
- Which reef tank parameter should I test first?
- Salinity, alkalinity, and temperature. Salinity and temperature are environmental controls every fish needs; alkalinity moves the fastest and stresses coral hardest when it drifts. Calcium, magnesium, nitrate, and phosphate become important once those three are stable.
- What's the difference between alkalinity, dKH, and KH?
- Functionally the same thing in reef hobby use. Alkalinity is the acid-neutralising capacity of the water — mostly bicarbonate and carbonate ions. dKH is German degrees of carbonate hardness, the unit hobby test kits report. KH is the same thing without the German prefix. Conversions: 1 dKH = 0.357 meq/L = 17.86 ppm CaCO₃.
- Why is zero nitrate not the goal?
- Undetectable nitrate starves the zooxanthellae that live inside coral tissue and removes a nitrogen source bacteria need to outcompete dinoflagellates. The 'ultra-low-nutrient' fashion of the early 2010s crashed a lot of tanks. Modern consensus (Randy Holmes-Farley, 2022 revision) targets 2–10 ppm NO₃ — measurably low, not zero.
- How often should I test reef parameters?
- Weekly: alkalinity, salinity, temperature (continuous via controller). Bi-weekly: calcium, nitrate, phosphate. Monthly: magnesium, pH (also continuous via controller). Test more frequently after any chemistry change or new livestock.
- What test kits are most accurate?
- Hanna Checkers (alkalinity, calcium, phosphate ULR) are the most precise photometric kits in the hobby band. Salifert kits are the titration standard — slower but reliable for calcium, magnesium, and alkalinity. Red Sea Pro test kits are the matched set if you're running their Reef Foundation supplement line. Avoid generic 'master test kit' bundles — accuracy is poor on the chemistry that matters.
- What's ICP testing and is it worth it?
- ICP-OES (inductively coupled plasma optical emission spectroscopy) measures 30+ elements in a 50 mL sample. Triton, ATI, Fauna Marin, and Oceamo are the main labs. It's worth running once or twice a year to catch trace-element drift (lithium, boron, strontium) and contamination (copper, aluminium) that hobby test kits miss. See the ICP testing guide for protocol details.
- Do freshwater tanks need the same parameters?
- No. Freshwater tanks care about pH, KH (general carbonate buffering), GH (general hardness, mostly Ca/Mg), temperature, and nitrate. They do not need to track calcium, magnesium, salinity, or phosphate in the same way reef tanks do — freshwater fish tolerate much wider chemistry swings than reef invertebrates.
- What's the most important rule across all parameters?
- Stability beats absolute set point. A reef tank held steady at 7.5 dKH year-round outperforms one that swings between 8 and 11 dKH every week. This is true for alkalinity, calcium, magnesium, pH, salinity, and temperature alike. Pick a target you can hold, then hold it.