Reef water parameters explained
What each number on your test kit means, the ranges to aim for, and why stability beats perfection.
Overview
Corals and fish don't need "perfect" water — they need stable water in a sensible range. This is a plain-English tour of the parameters worth testing, roughly where to keep them, and which ones matter most as you add corals.
The safety trio: ammonia, nitrite, nitrate
In an established tank, ammonia and nitrite should read zero — any reading is a warning sign. Nitrate is the end of the cycle; low is good, and you keep it in check with water changes and sensible feeding.
Salinity and temperature
Keep salinity steady (many reefers aim around 35PPT or 1.025 specific gravity) and top up evaporation with fresh RO water, never saltwater. Hold temperature stable in the 24-26 °C — a steady number matters more than the exact one.
The coral parameters: alkalinity, calcium, magnesium
As corals grow they use these up, so they're the ones to watch in a coral-heavy tank:
- Alkalinity (dKH) — the biggest lever on coral health and the one to keep most stable.
- Calcium — building material for coral skeletons.
- Magnesium — keeps calcium and alkalinity in balance.
Aim for widely-accepted reef ranges, and — more importantly — keep them steady. Big swings stress corals more than a value that's slightly off but stable.
How often to test
Test the safety trio while a tank is young, then settle into a routine — alkalinity most often in a coral tank, the others less frequently. Not sure how to read your results? Bring a water sample into the store and we'll go through it with you.
Common mistakes
- Chasing a single "ideal" number and causing swings.
- Topping up evaporation with saltwater (this raises salinity).
- Dosing additives without testing first.