Good reef tank water parameters are a salinity of 1.025 to 1.027 SG, alkalinity of 7.5 to 9.0 dKH, calcium of 390 to 450 ppm, magnesium of 1200 to 1450 ppm, nitrate of 5 to 20 ppm, phosphate of 0.05 to 0.20 ppm, pH of 7.9 to 8.4 and a temperature of 74 to 82 °F. These are sensible starting points, not rules. Every reef settles on its own numbers, and keeping those numbers steady matters more than landing on any exact value.
Reef tank parameters chart
The table below lists each parameter with a default ideal range and a suggested testing frequency. "New tank" means roughly the first three to six months after cycling, when livestock and demand are still changing. "Established" means a tank that has run steadily for several months with a consistent routine.
| Parameter | Ideal range | New tank | Established tank |
|---|---|---|---|
| Temperature | 74 to 82 °F | Check daily | Check daily (a controller or thermometer glance) |
| Salinity | 1.025 to 1.027 SG (about 35 ppt) | 2 to 3 times a week | Weekly, and after top-off changes |
| pH | 7.9 to 8.4 | 2 to 3 times a week | Weekly, or continuous with a probe |
| Alkalinity | 7.5 to 9.0 dKH | 2 to 3 times a week | 1 to 2 times a week |
| Calcium | 390 to 450 ppm | Weekly | Weekly to every 2 weeks |
| Magnesium | 1200 to 1450 ppm | Weekly | Every 2 to 4 weeks |
| Nitrate | 5 to 20 ppm | 2 times a week | Weekly |
| Phosphate | 0.05 to 0.20 ppm | 2 times a week | Weekly |
| Ammonia | 0 to 0.1 ppm | Daily during the cycle, then weekly | Only when something seems off |
| Nitrite | 0 to 1.5 ppm | During the cycle | Rarely needed |
| Potassium | 380 to 430 ppm | Monthly | Monthly to quarterly |
| Iodine | 0.05 to 0.08 ppm | Monthly | Monthly to quarterly |
| ORP | 250 to 450 mV | Continuous with a probe, if you have one | Continuous with a probe, if you have one |
| Silicate | 0.01 to 0.2 ppm | Check your source water | When you see diatoms or change filters |
| Copper | 0.0003 to 0.003 ppm (trace only) | Before adding invertebrates | Occasionally, or with a lab test |
The copper range is a natural trace level, not a treatment level; medication strength copper does not belong in a reef display. Nitrite matters while a tank cycles but is far less of a concern in salt water than in fresh, so most keepers stop testing it once the reef is established.
Salinity and temperature
Salinity and temperature set the baseline for everything else. Most reefs do well at 1.025 to 1.027 SG, which is about 35 ppt. The most common cause of drifting salinity is evaporation: water leaves, salt stays, and the reading creeps up. An auto top-off with fresh RO/DI water solves most of this.
Calibrate your refractometer or salinity probe with a proper calibration fluid rather than fresh water, since fresh water only checks zero. A reading that looks steady can still be off if the tool has drifted.
For temperature, pick a point inside 74 to 82 °F and aim to hold it within about 1 to 2 °F through the day. Many keepers settle around 77 to 79 °F. Daily swings are usually a bigger concern than the exact set point, so check how much your tank moves between lights on and lights off.
Alkalinity, calcium and magnesium
These three are the building blocks corals use to grow skeletons, and they are linked. Alkalinity is the one that moves fastest and the one corals tend to react to first, so it gets tested most often. A tank holding 8.07 dKH day after day is in a better place than one swinging between 7.6 and 9.0, even though both stay inside the ideal range.
Calcium usually follows alkalinity. If you dose both in balance, for example with two-part or kalkwasser, calcium tends to stay put and needs testing less often. Magnesium moves slowest of all, but low magnesium can make it harder to hold calcium and alkalinity steady, which is why it is worth checking every few weeks.
As corals grow, demand rises. A tank that needed little dosing at six months may need noticeably more at eighteen. Regular alkalinity tests show you that trend early, so you can adjust in small steps. Our guide to keeping alkalinity stable goes through measuring consumption and setting a dose.
Nitrate and phosphate
Nitrate and phosphate are nutrients. Corals and other photosynthetic life need some of each, and keeping them at zero is not the goal. A common starting point is 5 to 20 ppm nitrate and 0.05 to 0.20 ppm phosphate.
The ratio between them also matters to many keepers. If one sits at zero while the other stays measurable, algae and cyanobacteria can sometimes take advantage. How much weight to give the exact ratio is debated in the hobby, so treat it as one clue rather than a rule.
If either is climbing, start by looking at feeding, livestock load, and how your skimmer, refugium or water changes are keeping up. Tools such as GFO and carbon dosing can bring numbers down, but they work best when you lower things gradually over a week or two. Our guide on how to lower nitrate and phosphate covers the options and their trade-offs.
pH
A reef pH of 7.9 to 8.4 is normal, and pH naturally rises during the day and falls at night as photosynthesis starts and stops. A daily swing of a few tenths is expected.
Low pH in a reef is often linked to carbon dioxide in the room air, especially in tightly sealed homes, rather than to the tank itself. Options people use include fresh air near the skimmer intake, a CO2 scrubber, running a refugium on a reverse light schedule, or dosing kalkwasser. Alkalinity also supports pH, so a steady alkalinity reading helps here too.
If you use a pH probe, calibrate it regularly, since a slow drift can come from the probe as easily as the water.
Why stability beats chasing numbers
It is tempting to chase a favorite value with every reading. In practice, corals and fish usually handle a slightly unusual but steady number better than repeated corrections toward a "perfect" one.
A few habits help:
- Test at the same time of day. pH and alkalinity both shift over 24 hours, so consistent timing makes trends meaningful.
- Retest before you react. A single surprising result could be a test error. Confirm it, ideally with a fresh sample.
- Change one thing at a time. If you change dosing, feeding and lighting together, you will not know which change made the difference.
- Move slowly. Small daily adjustments are gentler than one large correction. For alkalinity, many keepers aim to move no more than about 1 dKH per day.
- Watch the trend, not the dot. A chart over several weeks tells you far more than any single test.
How fish-only, soft coral and SPS tanks differ
The ranges above suit a typical mixed reef, but the type of tank changes how much attention each parameter needs. Keepers disagree on some of these details, so take them as general patterns.
- Fish-only and fish with live rock. Fish are generally more forgiving of higher nitrate and phosphate, and calcium and alkalinity matter less without corals building skeletons. Salinity, temperature and ammonia remain the essentials. Many fish-only keepers tolerate nitrate above 20 ppm, though lower is still kinder to the system.
- Soft coral and LPS tanks. These often do well with nutrients toward the middle or upper part of the range, and they are usually more forgiving of small swings. Alkalinity and calcium still matter, especially as LPS colonies grow.
- SPS dominant tanks. Stony corals tend to be the most sensitive to changes in alkalinity, and many SPS keepers test alkalinity daily or rely on an automatic tester. Nutrients are often kept toward the lower end of the range, but running them too low is a known cause of pale or struggling SPS.
Whatever you keep, the best ideal range is the one your tank does well at and you can hold steady.
If your tank is brand new, ammonia and nitrite come first; see how to cycle a new reef tank before building the routine above.
Keeping track of your readings
Writing results down is what turns testing into understanding. In Reefability you can log each test, see it against your ideal ranges (or set your own), and view charts over time. If you run a Neptune Apex, Red Sea ReefBeat, HYDROS, GHL or similar controller, the app can read those values for you, and you can ask Reefer AI questions about your own tank data.
Frequently asked questions
What is the most important reef tank parameter to test?
For a reef with corals, alkalinity is usually the one to watch most closely, because it moves quickly and corals react to swings. Salinity and temperature come right alongside it as the foundation for everything else.
How often should I test my reef tank water?
In a new tank, test alkalinity, nitrate, phosphate, salinity and pH two to three times a week. Once the tank is established and stable, weekly testing of the main parameters is a common routine, with alkalinity more often in coral heavy tanks.
Do I need to hit the exact middle of each ideal range?
No. The ranges are starting points, and many healthy reefs sit near one end. What matters most is that your readings stay steady from week to week and that any changes you make are small and gradual.