Most healthy reef tanks run at 35 ppt salinity (1.026 specific gravity), 76-80°F, pH 7.8-8.5, alkalinity 7-11 dKH, calcium 380-450 ppm and magnesium 1,250-1,400 ppm. Ammonia and nitrite should be undetectable. Nitrate and phosphate are the only ones where good reefs differ a lot, from near zero in some SPS tanks to 20 ppm nitrate and 0.2 ppm phosphate in plenty of thriving mixed reefs.
Reef tank parameters chart #
The ranges below draw on Randy Holmes-Farley’s reef water parameters table and his updated nutrient target ranges. Natural seawater values are included for comparison.
| Parameter | Common reef target | Natural seawater | Unit |
|---|---|---|---|
| Salinity | 35 (SG 1.026) | 34-36 (SG 1.025-1.027) | ppt |
| Temperature | 76-80°F (24-27°C) | Varies by reef | °F / °C |
| pH | 7.8-8.5 (8.1-8.3 is better) | 8.0-8.3 | pH units |
| Alkalinity | 7-11 dKH (2.5-4 meq/L) | About 7 dKH | dKH |
| Calcium | 380-450 | About 420 | ppm |
| Magnesium | 1,250-1,400 | About 1,280 | ppm |
| Nitrate | About 2-10 for SPS; 5-50 acceptable in mature mixed reefs | Usually below 0.1 | ppm |
| Phosphate | About 0.03-0.1 for SPS; 0.06-0.3 in mature mixed reefs | About 0.005 | ppm |
| Ammonia | Undetectable (below 0.1) | Usually below 0.1 | ppm |
| Nitrite | Undetectable (below 0.2) | Near zero | ppm |
Randy’s 2004 table recommended far lower nutrients (nitrate below 0.2 ppm, phosphate below 0.03 ppm). He has since written that he’d let nitrate float between 5 and 50 ppm and phosphate between 0.06 and 0.3 ppm in a mature mixed reef. Both views are in use, and the right one depends on your corals and how you feed.
Does stability matter more than the exact number? #
For most parameters, yes. A tank that holds 8.0 dKH every day is usually better off than one that swings between 8.0 and 10.5, even though both stay “in range.” Corals adapt to the chemistry they live in, and sudden changes, especially in alkalinity and salinity, are behind a lot of unexplained tissue loss.
So pick a target inside each range and hold it. Change targets slowly, over weeks, if you change them at all.
Salinity and temperature #
Natural seawater averages about 35 ppt salinity, according to NOAA. That’s 1.026 specific gravity at typical tank temperatures, and it’s what most reef keepers target. Top off evaporation with fresh RO/DI water, never saltwater, because only water evaporates. For units and instruments, see what salinity should a reef tank be.
Temperature matters less than keeping it steady. Randy’s table allows 76-83°F. Most keepers choose a number between 76 and 80°F and use a heater and, in hot climates, a fan or chiller to hold it within a degree or two.
pH #
Reef tanks swing up during the day and down at night. A range of 7.8-8.5 is acceptable, 8.1-8.3 is better, and comparing readings at the same time of day is essential. Low pH usually means excess CO2, often from indoor air.
Alkalinity, calcium and magnesium #
These three work as a group:
- Alkalinity (7-11 dKH) is the one to watch most closely. Corals use it quickly, and swings cause trouble. Pick a number and hold it within about 0.5 dKH.
- Calcium (380-450 ppm) is used alongside alkalinity, at about 7 ppm of calcium per dKH of alkalinity consumed.
- Magnesium (1,250-1,400 ppm) keeps calcium and carbonate from precipitating. If calcium and alk won’t stay up, test magnesium.
If alkalinity is low, raising alkalinity with baking soda is the cheapest fix. Alk units trip people up: 1 meq/L equals 2.8 dKH, and 1 dKH equals 17.9 ppm expressed as calcium carbonate.
Nitrate and phosphate #
These are nutrients, not toxins, at the levels seen in reef tanks. The goal is enough to feed corals without fueling algae. SPS keepers usually run lean (nitrate 2-10 ppm, phosphate 0.03-0.1 ppm), while soft coral and LPS tanks often do fine higher. Letting either reach zero causes its own problems, including pale corals and outbreaks of cyanobacteria or dinoflagellates.
Ammonia and nitrite #
In an established tank, both should be undetectable. A reading usually means something died, the tank was overfed, or biological filtration was disturbed. During cycling, both rise and then fall to zero, and that’s how you know the cycle is done.
Do soft coral and SPS tanks need different parameters? #
The targets overlap more than people think. The differences are in how tightly you hold them.
| Tank type | Where it differs |
|---|---|
| Soft corals and LPS | Tolerate higher nutrients, and small alk swings matter less |
| SPS-dominated | Lean nutrients, tight alkalinity and salinity control |
| FOWLR (fish only with live rock) | Calcium, alk and Mg matter less; nitrate tolerance higher |
Fish-only tanks don’t need reef-level chemistry. Stable salinity, temperature, zero ammonia and nitrite, and nitrate kept under control are enough.
How do you use this chart? #
A chart is only useful next to your own numbers. Write down every test, and compare against the ranges above. Test frequency depends on the parameter and the tank’s age, which we cover in how often to test reef tank water. If some of these terms are new, aquarium water parameters explained covers what each one measures.
Fish Tank Tracker logs all ten of these parameters and draws each reading against an optimal band. Its built-in bands are 34-36 ppt salinity, 76-80°F, pH 8.0-8.3, alk 8-11 dKH, calcium 400-450, magnesium 1,300-1,400, nitrate 0-10 ppm and phosphate 0-0.1 ppm, with ammonia and nitrite at zero. Those nutrient bands sit at the lean end. If you deliberately run a mixed reef at 15 ppm nitrate, treat the color as a nudge and watch the trend line instead.
Frequently asked questions #
What is the most important reef tank parameter? #
Alkalinity stability is the one most reef keepers watch first, because corals use it fast and swings cause visible damage. Salinity is a close second, since evaporation changes it every day.
Is 1.025 specific gravity OK for a reef tank? #
Yes. 1.025 is roughly 33-34 ppt, a little below the 35 ppt most reefers target but still close to natural seawater. The bigger risk is letting salinity drift up and down between top-offs.
What should I test first in a new reef tank? #
Ammonia and nitrite first, until the cycle is complete. Then salinity, temperature, alkalinity and nitrate. Add calcium, magnesium and phosphate once you start keeping stony corals.
Can reef tank parameters be too perfect? #
Yes. Tanks pushed to zero nitrate and phosphate often run into pale corals and nuisance outbreaks. Stable numbers somewhere in the ranges above usually beat numbers that are theoretically ideal but hard to hold.