Key takeaways
- The "big five" for a reef tank are salinity, alkalinity, calcium, magnesium and nitrate/phosphate. Temperature and pH round out the picture.
- Test more often while a tank is new or after any change. Once it is stable, test to a routine.
- Alkalinity is the parameter that swings fastest in a coral-filled reef. Test it at least weekly, and every few days if you dose.
- Write every result down. Trends tell you far more than a single reading.
- Consider an ICP test every few months to check trace elements and pollutants that home kits can't measure.
Clear water does not always mean healthy water. Many of the problems reef keepers see, such as algae outbreaks, coral tissue loss or pale colours, start as a slow change in water chemistry. Regular testing lets you catch that change early. This guide explains what to test, how often, and what the numbers mean, so you can spend less time guessing and more time enjoying your reef.
Why water testing matters
Corals, fish and invertebrates are adapted to the stable conditions of a natural reef. In a closed aquarium, corals constantly use up alkalinity, calcium and magnesium as they grow. At the same time, fish food adds nitrate and phosphate. Without testing, these changes are invisible until something goes wrong. Testing turns your reef from guesswork into a system you can manage.
What to test in a reef tank
| Parameter | Typical reef target | Why it matters |
|---|---|---|
| Salinity | 35 ppt (≈1.025–1.026 SG) | Affects every animal's osmotic balance |
| Temperature | 25–26 °C | Stability prevents stress and bleaching |
| pH | 7.9–8.4 | Affects coral calcification |
| Alkalinity (KH) | 7–11 dKH, kept stable | Main building block for coral skeletons; swings fastest |
| Calcium | 400–450 ppm | Used with alkalinity for coral growth |
| Magnesium | 1,250–1,400 ppm | Keeps alkalinity and calcium in balance |
| Nitrate (NO₃) | 2–10 ppm (mixed reef) | Nutrient for corals; too much fuels algae |
| Phosphate (PO₄) | 0.03–0.1 ppm | Nutrient for corals; too much fuels algae |
| Ammonia / nitrite | 0 ppm | Toxic; essential to test while cycling |
These are common targets for a mixed reef. SPS-dominated systems are often kept at the tighter end of these ranges. For a full explanation of each parameter, see our reef water chemistry guide.
How often to test
Your testing routine depends on how old the tank is and what you keep:
| Parameter | New tank (first 3 months) | Established reef |
|---|---|---|
| Ammonia, nitrite | Every 2–3 days while cycling | Only if there's a problem |
| Salinity, temperature | Daily visual check, weekly test | Weekly |
| Alkalinity | Weekly | Weekly, or every 2–3 days if dosing |
| Calcium, magnesium | Fortnightly | Fortnightly to monthly |
| Nitrate, phosphate | Weekly | Weekly |
| pH | Weekly | Weekly, or continuously with a probe |
| ICP (trace elements) | After about 3 months | Every 3–6 months |
Also test whenever you notice a change, such as coral polyps not opening, new algae or fish behaving oddly, and after starting or changing any dosing.
Choosing your test kits
There are three main types of testing for home reef keepers:
- Titration and colour-comparison kits are affordable and accurate when used carefully. A good kit set is the foundation of reef testing. Browse Red Sea test kits or our full water testing range.
- Digital checkers and photometers take the guesswork out of reading colours. See the Hanna Instruments range.
- ICP testing: a laboratory analysis of a water sample that measures dozens of elements, including trace elements and contaminants such as metals. See our Fauna Marin home and ICP test kits.
Tips for accurate test results
- Test at the same time of day. pH and alkalinity naturally shift between day and night.
- Rinse vials with tank water, and never with tap water or soap.
- Follow the instructions exactly, including reagent drop counts, waiting times and lighting when reading colours.
- Check reagent expiry dates. Old reagents can give misleading results.
- Calibrate your refractometer regularly with calibration fluid, not RODI water alone.
- Log your results in a notebook, spreadsheet or app so you can spot trends.
What to do when a result is off
First, re-test to rule out a testing error. Then make changes slowly. Corals cope far better with a slightly "off" but stable parameter than with a fast correction. As a rule of thumb, don't raise alkalinity by more than about 1 dKH per day. For nutrients, look at the cause, such as feeding, filtration or water changes, before reaching for an additive. Consistent water changes with properly mixed salt are the simplest way to keep things in balance. Our step-by-step guide on how to mix saltwater for a reef tank shows you how.
Build your reef testing kit
Shop water test kits, Hanna digital checkers and ICP testing, delivered NZ-wide from our Auckland store. Bring your results into our Auckland store at 114 Wiri Station Road, and we're happy to help you interpret them.
Frequently asked questions
What is the most important parameter to test in a reef tank?
For a tank with corals, alkalinity is usually the most important to test regularly, because it is consumed quickly and corals react badly to swings. Salinity and temperature are close behind.
How often should I test my reef tank water?
Test salinity, alkalinity, nitrate and phosphate weekly in an established reef. Test calcium and magnesium every two to four weeks. Test more often in a new tank or after any change.
Do I need an ICP test?
It isn't essential for beginners, but an ICP test every three to six months is a valuable check on trace elements and contaminants that home kits can't detect.
Are digital testers better than test kits?
Digital checkers remove the guesswork of reading colours, which improves consistency. Well-made reagent kits are still accurate when used carefully. Many reefers use a mix of both.
