Ideal Reef Tank Parameters
There is no single "perfect" reef tank, but there is a band of water chemistry inside which corals, inverts and fish reliably thrive. What matters even more than hitting an exact number is holding each value steady day to day. A tank parked at alkalinity 9.5 forever beats one that bounces between 7 and 11. Below are the target ranges most reef-keepers aim for, and the reason each one matters.
Open ReefParams — track these for freeTarget ranges at a glance
| Parameter | Target range | Why |
|---|---|---|
| Alkalinity | 7–12 dKH | Carbonate buffer for skeleton growth & pH stability |
| Calcium | 380–450 ppm | Builds coral skeletons and coralline algae |
| Magnesium | 1250–1400 ppm | Keeps Ca and alkalinity in solution together |
| Salinity | 1.024–1.026 SG | Matches natural reef seawater (~35 ppt) |
| Temperature | 76–82 °F | Coral metabolism; stable beats precise |
| Nitrate | 2–10 ppm (under 25) | Some nutrients feed coral; too much fuels algae |
| Phosphate | 0.03–0.1 ppm | Trace nutrient; excess browns coral & grows nuisance algae |
The "big three": alkalinity, calcium, magnesium
These three move together and drive skeletal growth. Alkalinity (carbonate hardness) is the workhorse — it is consumed fastest and swings hardest, so it is the number to watch most closely. Calcium and alkalinity precipitate out if magnesium is low, which is why magnesium is held at roughly three times calcium. Keep alkalinity within about 0.5 dKH of your chosen point and you have solved most of reef chemistry.
Salinity and temperature
Salinity should sit around 1.025 specific gravity (35 ppt). Top off evaporation with fresh RO/DI water, never saltwater, or salinity will creep up. Temperature in the high 70s to low 80s Fahrenheit is fine; the danger is rapid change, so a heater plus a fan or chiller to cap summer spikes matters more than the exact set-point.
Nutrients: nitrate and phosphate
Modern reef-keeping no longer chases zero nutrients. A little nitrate and phosphate actually feeds coral tissue and zooxanthellae. The goal is a low but detectable level — roughly nitrate 2–10 ppm and phosphate 0.03–0.1 ppm — kept in proportion. Bottomed-out nutrients cause pale, starved coral and cyanobacteria; runaway nutrients cause algae and brown tissue.
Stability is the real target
Every range above is a starting point. SPS-dominant tanks often run tighter and lower-nutrient; softie and fish-only systems tolerate more drift. Whatever point you pick, the win is consistency — and the only way to see drift before it costs you a coral is to log every test. ReefParams shows your latest value, the trend versus last time, and flags anything out of range.
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