Reef Lab
Reef tank water parameters
Every value Reef Lab tracks, with the band it rates against — and, more usefully, what actually moves each one when it drifts. Guidance for a mixed reef tank.
| Parameter | Optimal | Marginal | Critical | Test |
|---|---|---|---|---|
| Alkalinity (KH) | 7.5–8.5 dKH | 7–9 | < 6 / > 10 | Weekly |
| Calcium | 420–440 mg/l | 400–450 | < 380 / > 480 | 2–4 weeks |
| Magnesium | 1300–1350 mg/l | 1250–1400 | < 1200 / > 1500 | 2–4 weeks |
| Nitrate | 5–10 mg/l | 2–15 | > 25 | 1–2 weeks |
| Phosphate | 0.05–0.08 mg/l | 0.03–0.1 | > 0.2 | 1–2 weeks |
| pH | 8.0–8.2 | 7.9–8.3 | < 7.7 / > 8.5 | Continuous or spot |
| Salinity | 1.024–1.026 sg | 1.023–1.027 | < 1.021 / > 1.029 | Every water change |
These are the ranges the app rates readings against, shown as a traffic light next to every entry. A pure SPS system runs tighter than this and a softie tank forgives more — but the ordering of priorities below holds for all of them.
Alkalinity — the one that moves
7.5–8.5 dKH. Test weekly, twice weekly in a busy SPS tank.
Alkalinity is the carbonate reserve corals build their skeletons from, and it is consumed continuously — fast enough that a tank with real coral mass can drop noticeably within a week. That makes it the parameter you test most and the one that punishes neglect fastest.
It is also the parameter most sensitive to speed. Corals respond to a fast swing with receding tissue and burnt tips, so a correction of more than about 1.4 dKH belongs spread across days rather than given in one evening. Stability inside the band beats hitting an exact figure.
Calcium — the other half
420–440 mg/l. Test every two to four weeks.
Calcium is consumed alongside alkalinity in a roughly fixed ratio, which is why the two are usually dosed as a pair. It moves more slowly in relative terms — 20 mg/l of calcium is a much smaller fraction of 430 than 0.5 dKH is of 8 — so it needs testing less often.
If calcium drops while alkalinity holds, or the other way round, something other than coral growth is at work: check magnesium, and check whether your dosing pair is actually balanced.
Magnesium — the enabler
1300–1350 mg/l. Test every two to four weeks.
Magnesium does nothing visible on its own, and that is exactly why it gets ignored until it causes trouble. Its job is to keep calcium and carbonate dissolved at concentrations where they would otherwise precipitate out.
Below roughly 1250 mg/l, alkalinity and calcium become impossible to hold: whatever you dose precipitates instead of staying in solution. If both keep falling despite dosing, test magnesium before you increase anything else.
Nitrate and phosphate — not enemies
5–10 mg/l and 0.05–0.08 mg/l. Test every one to two weeks.
The single most damaging piece of outdated advice in this hobby is that nutrients should read zero. Corals need them. Their symbiotic algae need nitrogen and phosphorus to function, and starving them produces pale, washed-out colonies, tissue loss, and very often a dinoflagellate outbreak — those thrive precisely where their competitors have been starved out.
Aim for measurable and modest. The ratio matters too: nitrate that bottoms out while phosphate stays up, or the reverse, tends to produce cyanobacteria and stubborn algae. Feeding more is a legitimate fix for low nutrients, and usually the cheapest one.
High values are a different conversation: more water changes, better skimming, a refugium, or simply less food — and always gradually, because dropping nutrients fast is as stressful as raising them fast.
pH — mostly a room problem
8.0–8.2, swinging daily. Worth a probe, not worth chasing.
pH rises during the day as photosynthesis consumes carbon dioxide and falls overnight as respiration releases it. A daily swing of around 0.2 is normal and healthy. What is worth investigating is a pH that sits low all day and night.
That is almost always carbon dioxide accumulating in the room rather than a fault in the water. Running fresh outside air to the skimmer intake, or simply opening a window, usually does more than any additive. Chasing pH with buffers instead treats the symptom and pushes alkalinity around as a side effect.
Salinity — the baseline
1.024–1.026 sg. Check with every water change.
Salinity is the reference every other test is implicitly measured against, so a wrong salinity quietly skews the whole picture. It also moves on its own: evaporation concentrates the tank daily, which is what an auto top-off exists to cancel.
Two practical points. Calibrate your refractometer with a 35 ppt seawater standard, not RO water — the difference can be a full ppt. And correct temperature, because the same water reads differently warm and cold.
Fix them in this order
Working out of order means fighting the same battle twice.
And the rule that outranks every number above: change one thing at a time, then wait. Two changes in the same week make the next test result impossible to interpret.
Questions
What are the ideal water parameters for a reef tank?
For a mixed reef tank: alkalinity 7.5 to 8.5 dKH, calcium 420 to 440 mg/l, magnesium 1300 to 1350 mg/l, nitrate 5 to 10 mg/l, phosphate 0.05 to 0.08 mg/l, pH 8.0 to 8.2 and salinity 1.024 to 1.026 specific gravity. These are guidance for a mixed reef; a pure SPS system runs tighter and a softie tank forgives more. Stability inside the band matters more than hitting any particular number.
How often should I test my reef tank water?
Alkalinity weekly, or twice a week in a heavily stocked SPS tank, because it moves fastest and matters most. Calcium and magnesium every two to four weeks. Nitrate and phosphate every one to two weeks, since they drive both algae and coral colour. Salinity whenever you mix water or notice evaporation behaving oddly. A new tank needs more frequent testing than an established one, and any tank needs more after a change.
Should nitrate and phosphate be zero in a reef tank?
No — that is one of the most damaging pieces of old advice still circulating. Corals need nutrients, and their symbiotic algae need nitrogen and phosphorus to function. Undetectable nitrate and phosphate cause pale, bleached-looking corals, tissue loss and often an outbreak of dinoflagellates, which thrive precisely where competitors have been starved out. Aim for measurable, modest values rather than zero.
Why does my pH swing during the day?
Photosynthesis consumes carbon dioxide during the day, which raises pH, and respiration releases it at night, which lowers it again. A daily swing of around 0.2 is completely normal and not worth chasing. A persistently low pH across the whole day usually means carbon dioxide has built up in the room rather than anything being wrong with the water — fresh air to the skimmer intake often fixes more than any additive.
Which parameter should I fix first?
Salinity, then magnesium, then alkalinity and calcium, then nutrients. Salinity is the baseline every other test is measured against, so a wrong salinity quietly distorts everything else. Magnesium comes next because low magnesium makes alkalinity and calcium impossible to hold no matter how much you dose. Fixing them in the other order means fighting the same battle repeatedly.
Put the numbers to work
Free calculators, no sign-up, same maths as the app.
Dosing calculator
Raise alkalinity, calcium or magnesium to a target, split over as many days as it takes.
Consumption calculator
Two readings, a few days apart — out comes the daily dose your pump should run.
Salinity calculator
Specific gravity to ppt with the temperature correction most converters skip.
These ranges, applied to your tank automatically
Reef Lab rates every reading you log against exactly these bands and shows the trend next to it — so you see a value drifting before it becomes a problem. Free, works offline, no account to start.
Reef Lab does not replace professional or veterinary advice. When something is genuinely wrong, your local store, your vet or a lab analysis has the last word.