For a mixed reef: alkalinity 8–9.5 dKH, calcium 400–450 ppm, magnesium 1,300–1,400 ppm, nitrate 5–10 ppm, phosphate 0.03–0.10 ppm, 35 ppt, 77–80 °F. Two things the chart below says that most do not: nitrate and phosphate have a floor, not just a ceiling — and calcium, magnesium and salinity are the same whatever corals you keep.
The chart
Four tank types across, natural seawater on the end for comparison. Print it and put it wherever you keep the test kits.
| Parameter | SPS, low nutrient | Mixed reef | LPS and soft coral | Fish only (FOWLR) | Natural seawater |
|---|---|---|---|---|---|
| Alkalinity dKH | 7.0–8.5 | 8.0–9.5 | 8.0–11.0 | 7.0–11.0 | 7 |
| Calcium ppm | 400–450 | 400–450 | 400–450 | 380–450 | 420 |
| Magnesium ppm | 1,300–1,400 | 1,300–1,400 | 1,300–1,400 | 1,250–1,400 | 1,280 |
| Nitrate ppm | 2–5 | 5–10 | 5–20 | under 40 | 0.1 |
| Phosphate ppm | 0.02–0.05 | 0.03–0.10 | 0.05–0.15 | under 0.50 | 0.005 |
| Salinity ppt | 34–35 | 34–35 | 34–35 | 33–35 | 35 |
| Temperature °C | 25.0–26.5 | 25.0–26.5 | 25.0–27.0 | 25.0–27.0 | — |
| Temperature °F | 77–80 | 77–80 | 77–81 | 77–81 | — |
| pH | 8.0–8.4 | 8.0–8.4 | 7.9–8.4 | 7.9–8.4 | 8.2 |
Hobby-consensus ranges; alkalinity, calcium and magnesium come from this site's reef engine, so they cannot drift from the dosing calculators. Nitrate and phosphate have a floor as well as a ceiling — zero is a failure state for a coral tank, not a goal. Salinity in ppt; use the salinity converter for what your own instrument should read.
The last column is on the chart because the gaps are deliberate and worth seeing. Ocean alkalinity is about 7 dKH and we run a mixed reef at 8–9.5 — not because more is better, but because a closed system with heavy calcification needs a buffer against a reading falling between doses. The reef also runs magnesium above ocean levels, which helps keep calcium and alkalinity in solution together at concentrations that would otherwise precipitate. Nutrients are the interesting one: the ocean is far poorer in nitrate and phosphate than any tank on this chart, and corals there are fed by a constant flow of plankton that your tank does not have.
These are not eight independent dials
Read the chart across rather than down and it answers a question it was not asked. Of the eight parameters, 3 of them — calcium, magnesium and salinity — are identical for every coral tank on it. They do not vary with what you keep. Neither, in practice, do temperature or pH.
That matters because the usual presentation — eight rows, one per number, each with its own section — implies eight things to tune. There are not eight. There is one target band for the big three, one decision about nutrients, and a great deal of attention to keeping all of it still.
The big three are not independent of each other either. Corals build skeleton from calcium and carbonate together, in a fixed ratio: every 1 dKH of alkalinity consumed comes with about 7.2 ppm of calcium. So the two should fall together, and if yours do not, the useful conclusion is usually about a test kit rather than about the tank. The two-part calculator checks that ratio against your own two test points and says which way it is off.
The row that actually differs: nutrients
So "targets by coral type" really comes down to one decision. Across the three coral tanks on the chart, nitrate runs from 2–5 at the SPS end to 5–20 for LPS and softies, and phosphate from 0.02–0.05 to 0.05–0.15. Everything else on the chart stayed still.
And the direction of the error is worth naming, because the hobby spent roughly a decade going the wrong way. The ultra-low-nutrient era treated undetectable nitrate and phosphate as the goal, and a great many charts still carry a zero in those rows. It does not work, for a reason that is obvious once stated: corals are animals hosting photosynthetic algae, and both need nitrogen and phosphorus. Strip those out and the coral pales, growth stops, and the tank becomes hospitable to exactly the organisms that can fix their own nitrogen — dinoflagellates and cyanobacteria, which is why "spotless" tanks so often end up covered in them.
Not a target reached. If nitrate reads undetectable and phosphate reads zero in a coral tank, the correct response is to feed more — the fish, and the corals directly — and to ease off whatever export you are running: less skimming, less carbon dosing, fewer absorbent media. Pale tips and closed polyps in an immaculate tank are a starvation symptom, and they look nothing like a disease because they are not one.
Stability, in numbers
Every chart says stability beats the set point and none of them says how much movement is too much. Here is the version with numbers in it.
Your tank consumes alkalinity continuously and you replace it in doses. The gap between those two facts is your swing. A tank using 0.5 dKH a day, dosed once daily, moves the full 0.5 dKH between doses. Split the same total across more doses and the swing divides:
| Doses per day | Alkalinity swing | Calcium swing | |
|---|---|---|---|
| 1 | 0.5 dKH | 3.6 ppm | A visible daily cycle. Fine for softies, poor for SPS. |
| 2 | 0.25 dKH | 1.8 ppm | Better, and about the limit of doing it by hand. |
| 4 | 0.125 dKH | 0.9 ppm | A dosing pump on a timer. The usual answer. |
| Continuous (hourly) | 0.021 dKH | 0.1 ppm | What a calcium reactor or a well-set pump achieves. |
A tank consuming 0.5 dKH per day, with the calcium column derived from the same 7.16 ppm per dKH ratio the corals themselves use. For comparison, the safe correction rate when you are deliberately moving alkalinity is 1 dKH per day — a swing approaching that every day is not stability, whatever the average says.
Which of these to actually watch
- Salinity and temperature. First because they are the foundation everything else is measured against — a salinity error makes every other test wrong at once — and because both are set-and-forget with decent equipment.
- Alkalinity. The fastest-moving number in the tank and the one whose swings do visible damage. If you test one thing regularly, test this.
- Nitrate and phosphate. Slow, and the pair that decides how your corals look rather than whether they survive. Watch the trend, not the reading.
- Calcium and magnesium. Large reservoirs that move slowly; genuinely important and rarely urgent. Monthly is plenty unless something else has gone wrong.
- pH. Last, and mostly a readout of your house rather than your husbandry. Worth understanding, rarely worth chasing.
How often to test
| Tank | Alk | Ca / Mg | NO₃ / PO₄ |
|---|---|---|---|
| New, first 2 months | Every 2–3 days | Weekly | Weekly |
| SPS dominant | 2–3× a week | Monthly | Weekly |
| Mixed reef | Weekly | Monthly | Every 2 weeks |
| LPS and softies | Every 1–2 weeks | Monthly | Every 2 weeks |
| Fish only | Rarely | Rarely | Monthly |
Plus a full set after anything unusual: a new addition, a dosing change, a heater failure, a holiday. Testing more often than this mostly measures your test kit.
When one of them is off, these do the arithmetic:
- Alkalinity — Alkalinity Dosing
- Calcium — Calcium Dosing
- Magnesium — Magnesium Dosing
- All three, daily — Two-Part Dosing
- Nitrate and phosphate — Water Change Correction
- Salinity — Salinity Adjustment
What you'll be told, and what's true
- "Zero nitrate and zero phosphate means a clean tank." It means a starving one. Corals need both, and undetectable nutrients are the condition dinoflagellates and cyanobacteria exploit best. Every coral band on this chart has a floor above zero for that reason.
- "Higher alkalinity grows coral faster." There is something to it — calcification does respond to carbonate availability — but it only holds where light and nutrients can support the extra growth, and the margin for error shrinks as you go up. High and wandering is the worst combination available, and it is what chasing growth usually produces.
- "Match natural seawater and you can't go wrong." Reasonable for salinity, calcium and temperature. Not for alkalinity, where a closed system wants a buffer the ocean does not need, and not for nutrients, where ocean-level scarcity comes packaged with a constant supply of plankton your tank has no equivalent of.
- "Test everything every day and correct what's off." That is how tanks end up on a rollercoaster. Most kits have a margin of error comparable to the daily movement you are trying to detect, so daily correcting chases test-kit noise. Watch the trend across several readings and change one thing at a time.
Common questions
What alkalinity should I run?
Pick a number inside your tank's band and hold it there. For a mixed reef that is 8–9.5 dKH; for an SPS tank, 7–8.5. Which number inside the band matters far less than whether it moves — a tank sitting at 8.2 every day is in better shape than one averaging 9.0 while wandering a full point either side.
Is zero nitrate and zero phosphate a good thing?
No, and this is the single most damaging idea still circulating in reef charts. Corals need nitrogen and phosphorus; their symbiotic algae are living organisms with nutritional requirements. Tanks driven to undetectable nutrients go pale, stop growing, and are the ones that end up fighting dinoflagellates and cyanobacteria, which thrive precisely where nothing else can compete. Every coral band on this chart has a floor above zero on purpose.
Do I need to dose magnesium?
Usually not on any schedule. Magnesium sits in the tank at roughly three times the concentration of calcium and is consumed far more slowly, so a normal water change routine replaces most of what goes. Test it monthly, and take it seriously when alkalinity or calcium refuse to hold despite dosing — low magnesium is the classic reason both become unstable at once.
My pH reads 7.8 — is that a problem?
Usually it is your house, not your tank. Indoor CO₂ builds up in a closed home and dissolves into the water, and that is by far the most common cause of a reef running low on pH. The effective fixes are ventilation, an air line to the skimmer drawing from outside, or a refugium lit at night — not pH-adjusting chemicals, which move alkalinity around to chase a number that swings on its own every day anyway.