Water chemistry is the foundation of every successful reef tank. Understanding what to test, why it matters, and how to maintain stability will save you from most common problems.
The Big Three: Alkalinity, Calcium, and Magnesium
These three parameters are interconnected and critical for coral growth.
Alkalinity (dKH)
Target: 7–9 dKH
Alkalinity is the most important parameter to monitor. It measures your water's buffering capacity and is consumed rapidly by growing corals. Low alkalinity causes pH swings, slow coral growth, and tissue recession. High alkalinity can cause tissue burn and rapid tissue necrosis (RTN).
Test alkalinity at least twice a week. Once you know your daily consumption, you can dose accordingly.
Calcium (Ca)
Target: 400–450 ppm
Calcium is the building block of coral skeletons. It works in tandem with alkalinity — if one is off, the other usually is too. Most reef tanks consume 10–20 ppm of calcium per day.
Magnesium (Mg)
Target: 1250–1350 ppm
Magnesium is the unsung hero. It prevents calcium and alkalinity from precipitating out of solution. If you can't keep your calcium and alkalinity stable, check magnesium first — it's often the culprit.
Other Critical Parameters
Salinity
Target: 1.025–1.026 sg (35 ppt)
Use a refractometer, not a hydrometer. Calibrate it with calibration fluid regularly. Top off evaporation with RO/DI freshwater daily — an auto top-off (ATO) system is a worthwhile investment.
Temperature
Target: 25–26°C
Stability matters more than the exact number. Fluctuations of more than 1°C per day stress livestock. Use a quality heater with a reliable thermostat, and consider a controller for peace of mind.
pH
Target: 8.0–8.3
pH naturally fluctuates throughout the day (lower at night, higher during the day when corals photosynthesise). Don't chase pH — if your alkalinity is stable, pH usually takes care of itself. Opening a window for fresh air exchange can help raise pH in sealed-up houses.
Nitrate (NO₃)
Target: 5–15 ppm for mixed reefs
Zero nitrate isn't ideal — corals actually need some nitrate for nutrition. But above 20–30 ppm, you'll see nuisance algae and coral stress. Control nitrate through water changes, protein skimming, and carbon dosing if needed.
Phosphate (PO₄)
Target: 0.03–0.10 ppm
Like nitrate, some phosphate is beneficial, but excess feeds algae. GFO (granular ferric oxide) media can help control phosphate, but use it carefully — dropping phosphate too quickly causes coral bleaching.
Testing Schedule
| Parameter | Frequency |
|---|---|
| Alkalinity | 2–3x per week |
| Calcium | Weekly |
| Magnesium | Fortnightly |
| Salinity | Daily (or use ATO) |
| Temperature | Continuous (use a thermometer) |
| Nitrate | Weekly |
| Phosphate | Weekly |
| pH | Optional if alk is stable |
| Ammonia/Nitrite | Only when troubleshooting |
Dosing
Once your corals start consuming alkalinity and calcium faster than water changes can replenish, you'll need to dose. Options include:
- Two-part dosing (e.g., B-Ionic, ESV) — simplest method, good for small to medium tanks
- Kalkwasser — calcium hydroxide mixed into top-off water, also boosts pH
- Calcium reactor — dissolves calcium media using CO₂, best for large or heavily stocked tanks
The Most Important Rule
Consistency beats perfection. A tank that sits at 7.5 dKH every day will grow better coral than one that swings between 8 and 10 dKH. Pick a target, dose consistently, and test regularly.
Free Water Testing
Bring a water sample into Eastwood Aquarium anytime and we'll test it for free. We use lab-grade equipment and can help you interpret the results and adjust your dosing.