Most reef problems announce themselves long before you see them. A coral that stops extending its polyps, a patch of algae that will not shift, a tank that looks flat and grey: the cause is usually a number that drifted while you were busy. Testing is not about chasing perfect figures. It is about knowing what your tank is doing so you can nudge it back before anything suffers.
Here is what each key parameter actually does, roughly where you want it, and why it matters.
Salinity: the base everything else sits on
Salinity is the total dissolved salt content of your water, and it changes how every other reading should be interpreted. Measure it with a refractometer or a conductivity meter, and calibrate that device with a proper reference solution rather than tap water. Tap water calibration is one of the most common reasons reef keepers run at the wrong salinity for months without realising.
Natural seawater sits at about 35 ppt, which most refractometers show as a specific gravity of 1.026 at 25°C. A practical reef range is 34–36 ppt, or 1.024–1.027. Corals cope with a slow drift far better than a fast one, so top up evaporation with fresh RO water at least daily, ideally with an auto top-off unit, and mix new saltwater to the same figure every time. Never use saltwater for top-ups; only fresh water evaporates.
Alkalinity, calcium and magnesium: the big three
These three work as a team. Corals and coralline algae build skeletons from calcium and carbonate, magnesium keeps the balance in solution, and alkalinity measures the carbonate buffer available to them. Push one out of line and the other two become much harder to hold steady.
Alkalinity
Measured in dKH or meq/L, alkalinity is the parameter you will test most often. It moves quickly, because consumption is high and because it responds to dosing errors, low magnesium and even the CO2 levels in the room. Natural seawater is around 7 dKH; most reef keepers run between 7.5 and 9 dKH. Stability matters more than the exact figure, so a tank sitting at 8.5 dKH all month is healthier than one swinging between 6 and 11.
Test at the same time each day. Alkalinity dips through the afternoon and climbs overnight as photosynthesis and respiration shift the tank's CO2 balance, so comparing a morning reading with an evening one tells you very little.
Calcium
Aim for 400–450 ppm, with 420–430 a comfortable target. Calcium moves slowly, which is why newer keepers dose it heavily and then wonder why alkalinity will not hold. If calcium sits above 450 and alkalinity keeps falling, stop dosing calcium and let it come down on its own. As a rough guide, every 1 dKH of alkalinity your tank consumes uses around 7 ppm of calcium, which is a useful ratio to remember when you start dosing.
Magnesium
Magnesium should sit around 1280–1350 ppm. Its role is quiet but essential: without enough of it, calcium and carbonate can precipitate out of solution, and you will find yourself adding more and more alkalinity for no gain. Low magnesium is a classic hidden cause of a tank that refuses to stabilise. Test it occasionally and raise it slowly if needed.
Nitrate and phosphate: the nutrients that decide how the tank looks
Nitrate and phosphate are not simply waste. They feed the zooxanthellae inside your corals, and a tank stripped to zero tends to look pale, washed out and slow to grow. The trick is keeping them low without bottoming them out.
- Nitrate: 0.5–5 ppm suits most mixed reefs. SPS-dominated tanks often run at the lower end, while soft coral tanks tolerate more.
- Phosphate: 0.03–0.10 ppm is a sensible band. Below 0.02 ppm you risk starving corals; above 0.1 ppm algae finds life much easier.
- The balance matters: roughly 100 parts nitrate to 1 part phosphate. A tank with zero nitrate and stubborn phosphate usually needs more nitrate, not another phosphate remover.
If both climb together, look at feeding, stocking and filtration before buying anything. Cut back on frozen food, rinse it before feeding, add a refugium or improve skimming, then give it a few weeks. Chasing a number with a bottle is rarely the answer.
How often to test, and how to test well
Frequency depends on your tank, but a workable baseline looks like this:
- Salinity: every time you mix water, plus a quick weekly check.
- Alkalinity: two or three times a week while dosing, daily when tuning a new dose.
- Calcium and magnesium: weekly, and after any change to your dosing.
- Nitrate and phosphate: weekly.
- A full suite or ICP test every few months, to catch what home kits cannot measure.
Technique matters as much as frequency. Rinse vials with tank water, compare colour charts under the same lighting each time, keep reagent bottles closed, and replace kits before they expire. A modest kit used carefully beats an expensive one used sloppily.
What to do when a reading drifts
Change one thing at a time, and change it slowly. A rise of 1 dKH per day is acceptable; 3 dKH is not. If a number looks impossible, retest before acting. Contaminated vials, air bubbles and a forgotten dose are far more common than genuine swings.
Write your readings down, on paper or in an app. Trends tell you far more than single results. Three weeks of alkalinity gently falling means demand is rising and it is time to increase the dose; one odd number means very little on its own.
A simple routine that works
Keep salinity steady at 35 ppt, hold alkalinity inside a one-point window, keep calcium and magnesium in range, and let nitrate and phosphate sit low without hitting zero. That is genuinely most of reef keeping.
Test at the same time of day, log the results, and resist making big corrections after one bad reading. If something stays wrong and you cannot find the cause, ask in a reefing community or consider an ICP test before changing equipment or dosing. Your corals will tell you the rest.
Photo: EddieKphoto / Pixabay
