Key takeaways
- Stable water chemistry matters more than chasing perfect numbers.
- Know your core parameters: salinity, temperature, pH, alkalinity, calcium, magnesium, nitrate and phosphate. Ammonia and nitrite should always read zero once a tank has cycled.
- Alkalinity, calcium and magnesium are used up by corals and need topping up through water changes or dosing.
- Make every correction slowly, and re-test before you act.
Your reef lives in its water. Fish, corals and invertebrates all depend on a narrow set of conditions, and most problems in a reef tank, such as algae, pale corals and stressed fish, can be traced back to water chemistry. This guide explains the numbers that matter, what each one does and how to keep them stable.
Reef tank parameters at a glance
| Parameter | Typical reef target | Test how often (established tank) |
|---|---|---|
| Salinity | 35 ppt (≈1.025–1.026 SG) | Weekly |
| Temperature | 25–26 °C | Daily glance at the thermometer |
| pH | 7.9–8.4 | Weekly |
| Alkalinity | 7–11 dKH (stable) | Weekly, or more often if dosing |
| Calcium | 400–450 ppm | Fortnightly |
| Magnesium | 1,250–1,400 ppm | Fortnightly to monthly |
| Nitrate | 2–10 ppm (mixed reef) | Weekly |
| Phosphate | 0.03–0.1 ppm | Weekly |
| Ammonia / nitrite | 0 ppm | While cycling, or if there's a problem |
For a step-by-step testing routine, see Reef Tank Water Testing: What to Test and How Often.
Salinity
Salinity is the amount of dissolved salt in the water. Marine animals are adapted to around 35 parts per thousand (ppt), which is about 1.025–1.026 specific gravity. Large or rapid changes cause osmotic stress.
Measure it with a calibrated refractometer or digital meter. Evaporation removes only water, so salinity rises as water evaporates. Top up with fresh RODI water, never saltwater. An auto top-off system makes this easy. Learn how to make up new water correctly in our guide How to Mix Saltwater for a Reef Tank.
Temperature
Most reef tanks are kept at 25–26 °C. Stability is the goal: aim to keep daily swings within about 1 °C. Use a reliable heater sized for your tank. In warmer parts of New Zealand, you may also need a fan or chiller over summer. Browse our heaters and chillers.
pH
pH measures how acidic or alkaline the water is. Reef tanks usually sit between about 7.9 and 8.4, and pH naturally drops at night as carbon dioxide builds up. Low pH is often caused by high carbon dioxide indoors or low alkalinity. Good gas exchange at the water surface and stable alkalinity usually keep it in range.
Alkalinity, calcium and magnesium
These three are the building blocks corals and coralline algae use to make their skeletons:
- Alkalinity (dKH) buffers pH and is used fastest by growing corals. It is the most important one to keep stable. Avoid changing it by more than about 1 dKH per day.
- Calcium is used alongside alkalinity. Keep it around 400–450 ppm.
- Magnesium helps keep calcium and alkalinity in balance. If it is low, it is hard to keep alkalinity and calcium stable.
In a lightly stocked tank, regular water changes may replace what corals use. As corals grow, most reefers add a dosing routine such as a two-part, four-part or balling system. Browse our additives and treatments and the Red Sea Reef Care range.
The nitrogen cycle: ammonia, nitrite and nitrate
Fish waste and uneaten food break down into ammonia, which is highly toxic. Beneficial bacteria in your rock and filter convert ammonia into nitrite (also toxic) and then into nitrate, which is far less harmful. A new tank must "cycle" to build up these bacteria before you add livestock. Once cycled, ammonia and nitrite should always test at zero.
Nitrate builds up over time. A little is useful food for corals, but too much fuels algae. Control it with sensible feeding, regular water changes, a protein skimmer and, where needed, filter media.
Phosphate
Phosphate also comes mainly from food. Like nitrate, a small amount supports coral health and colour, while too much leads to nuisance algae and can slow coral growth. If phosphate is high, reduce feeding, check your RODI water reads 0 TDS and consider a phosphate-removing medium. Lower it gradually.
Keeping chemistry stable
- Test on a regular schedule and log your results.
- Use RODI water and a quality reef salt for every water change.
- Change 10–20% of the water every one to two weeks.
- Make one change at a time and give the tank time to respond.
- Consider an ICP test every few months to check trace elements and contaminants. See our Fauna Marin ICP test kits.
Everything you need for stable reef chemistry
Shop water testing, reef salt and RODI units, delivered NZ-wide from our Auckland store. Visit us at 114 Wiri Station Road, Wiri, if you'd like help interpreting your results.
Frequently asked questions
What is the most important water parameter in a reef tank?
Once the tank has cycled, alkalinity stability is usually the most important for corals, followed closely by salinity and temperature.
Why is my nitrate zero?
Very low nitrate can mean your filtration is removing it faster than it is produced. Some reefers find corals look pale when both nitrate and phosphate bottom out. Re-test with a reliable kit before making changes.
How quickly can I correct a parameter?
Slowly. For alkalinity, aim for no more than about 1 dKH per day. For salinity, make small changes over several days.
