ReefBay data · Real reef tank water parameters
What are normal water parameters in a real reef tank?
Updated November 2, 2026 · Sample: 5,947 tanks · Recomputed weekly
Most parameter guides repeat textbook targets. This page shows what reef tanks actually run, from readings logged in the ReefBay app. Each tank counts once per parameter, using its latest reading in the last 12 months.
Key findings
The median reef tank in ReefBay runs 8.3 dKH alkalinity. The middle half of tanks run between 7.7 and 9.3 dKH (4,631 tanks, latest reading in the last 12 months).
The median reef tank has 7.0 ppm nitrate; the middle half sit between 2.0 and 15.0 ppm. 16.6% of tanks run above 20 ppm.
The median reef tank has 0.100 ppm phosphate; the middle half sit between 0.030 and 0.240 ppm. 34.7% of tanks run above 0.15 ppm.
The median reef tank runs 430 ppm calcium and 1,350 ppm magnesium.
The median reef tank runs 33.7 ppt salinity (about 1.025 specific gravity).
The median reef tank runs at 78.0 °F (25.6 °C) with a pH of 8.00.
What reef tanks actually run
Latest reading per tank in the last 365 days, from 341,976 logged readings across 5,947 tanks. "Typical range" is the middle half of tanks (25th to 75th percentile).
| Parameter | Median | Typical range | 10th to 90th | Common target | In target | Tanks |
|---|---|---|---|---|---|---|
| Alkalinity | 8.3 dKH | 7.7 to 9.3 | 7.0 to 10.4 | 7.5 to 11.0 | 75.9% | 4,631 |
| Calcium | 430 ppm | 410 to 460 | 385 to 499 | 380 to 450 | 65.7% | 4,032 |
| Magnesium | 1,350 ppm | 1,270 to 1,410 | 1,200 to 1,480 | 1,250 to 1,400 | 54% | 3,540 |
| Nitrate | 7.0 ppm | 2.0 to 15.0 | 0.0 to 28.8 | 1.0 to 20.0 | 63.2% | 4,877 |
| Phosphate | 0.100 ppm | 0.030 to 0.240 | 0.010 to 0.500 | 0.020 to 0.150 | 52.9% | 4,308 |
| Salinity | 33.7 ppt | 33.7 to 35.0 | 32.3 to 35.0 | 33.0 to 36.0 | 80.4% | 4,460 |
| Temperature | 78.0 °F | 77.0 to 78.6 | 76.6 to 79.5 | 76.0 to 80.0 | 90.1% | 3,727 |
| pH | 8.00 | 8.00 to 8.20 | 7.80 to 8.30 | 7.80 to 8.40 | 92.5% | 3,596 |
Share of tanks inside, above and below the common target
"Common target" is the range most reef-keeping guides recommend. It is shown for comparison, not as advice. The number in the middle is the median.
Alkalinity
- Below 19%
- In target 76%
- Above 5.4%
Calcium
- Below 6.9%
- In target 66%
- Above 27%
Magnesium
- Below 19%
- In target 54%
- Above 27%
Nitrate
- Below 20%
- In target 63%
- Above 17%
Phosphate
- Below 12%
- In target 53%
- Above 35%
Salinity
- Below 15%
- In target 80%
- Above 4.8%
Temperature
- Below 4.7%
- In target 90%
- Above 5.2%
pH
- Below 4.5%
- In target 93%
- Above 3.0%
Median by tank size
Nano = 30 gallons or less. Mid = 31 to 100 gallons. Large = more than 100 gallons.
| Parameter | Nano | Mid | Large |
|---|---|---|---|
| Alkalinity | 8.3 dKH (1,535) | 8.4 dKH (2,327) | 8.4 dKH (769) |
| Calcium | 430 ppm (1,271) | 430 ppm (2,072) | 431 ppm (689) |
| Magnesium | 1,320 ppm (1,063) | 1,348 ppm (1,845) | 1,350 ppm (632) |
| Nitrate | 5.7 ppm (1,680) | 7.3 ppm (2,417) | 9.3 ppm (780) |
| Phosphate | 0.080 ppm (1,401) | 0.100 ppm (2,173) | 0.100 ppm (734) |
| Salinity | 33.7 ppt (1,559) | 33.7 ppt (2,219) | 33.7 ppt (682) |
| Temperature | 78.0 °F (1,307) | 78.0 °F (1,845) | 77.9 °F (575) |
| pH | 8.00 (1,207) | 8.01 (1,832) | 8.10 (557) |
Alkalinity distribution
4,631 tanks. Median 8.3 dKH. 27 readings outside 4.0 to 16.0 dKH were dropped.
- 4.0 to 5.0 dKH 0.4%
- 5.0 to 6.0 dKH 1.7%
- 6.0 to 7.0 dKH 7.1%
- 7.0 to 8.0 dKH 22.2%
- 8.0 to 9.0 dKH 36.8%
- 9.0 to 10.0 dKH 17.3%
- 10.0 to 11.0 dKH 8.1%
- 11.0 to 12.0 dKH 3.4%
- 12.0 to 13.0 dKH 1.6%
- 13.0 to 14.0 dKH 0.8%
- 14.0 to 15.0 dKH 0.6%
- 15.0 to 16.0 dKH 0.2%
Calcium distribution
4,032 tanks. Median 430 ppm. 56 readings outside 300 to 600 ppm were dropped.
- 300 to 325 ppm 0.9%
- 325 to 350 ppm 1.3%
- 350 to 375 ppm 3.7%
- 375 to 400 ppm 9.3%
- 400 to 425 ppm 27.1%
- 425 to 450 ppm 23%
- 450 to 475 ppm 16.8%
- 475 to 500 ppm 7.9%
- 500 to 525 ppm 4.9%
- 525 to 550 ppm 1.8%
- 550 to 575 ppm 1.3%
- 575 to 600 ppm 1.9%
Magnesium distribution
3,540 tanks. Median 1,350 ppm. 70 readings outside 1,000 to 1,700 ppm were dropped.
- 1,000 to 1,058 ppm 1.2%
- 1,058 to 1,117 ppm 2%
- 1,117 to 1,175 ppm 4.6%
- 1,175 to 1,233 ppm 9.2%
- 1,233 to 1,292 ppm 17.1%
- 1,292 to 1,350 ppm 15.8%
- 1,350 to 1,408 ppm 23.5%
- 1,408 to 1,467 ppm 13.3%
- 1,467 to 1,525 ppm 8.7%
- 1,525 to 1,583 ppm 2.3%
- 1,583 to 1,642 ppm 1.6%
- 1,642 to 1,700 ppm 0.5%
Nitrate distribution
4,877 tanks. Median 7.0 ppm. 14 readings outside 0.0 to 100.0 ppm were dropped.
- 0.0 to 8.3 ppm 53.8%
- 8.3 to 16.7 ppm 23.7%
- 16.7 to 25.0 ppm 8.3%
- 25.0 to 33.3 ppm 6.4%
- 33.3 to 41.7 ppm 2.7%
- 41.7 to 50.0 ppm 1%
- 50.0 to 58.3 ppm 2.1%
- 58.3 to 66.7 ppm 0.5%
- 66.7 to 75.0 ppm 0.3%
- 75.0 to 83.3 ppm 0.7%
- 83.3 to 91.7 ppm 0.1%
- 91.7 to 100.0 ppm 0.3%
Phosphate distribution
4,308 tanks. Median 0.100 ppm. 47 readings outside 0.000 to 2.000 ppm were dropped.
- 0.000 to 0.167 ppm 67%
- 0.167 to 0.333 ppm 17.8%
- 0.333 to 0.500 ppm 4.8%
- 0.500 to 0.667 ppm 4.2%
- 0.667 to 0.833 ppm 1.4%
- 0.833 to 1.000 ppm 1.9%
- 1.000 to 1.167 ppm 1.7%
- 1.167 to 1.333 ppm 0.4%
- 1.333 to 1.500 ppm 0.1%
- 1.500 to 1.667 ppm 0.1%
- 1.667 to 1.833 ppm 0.2%
- 1.833 to 2.000 ppm 0.4%
Salinity distribution
4,460 tanks. Median 33.7 ppt. 45 readings outside 28.0 to 40.0 ppt were dropped.
- 28.0 to 29.0 ppt 0.6%
- 29.0 to 30.0 ppt 0.8%
- 30.0 to 31.0 ppt 3.6%
- 31.0 to 32.0 ppt 0.2%
- 32.0 to 33.0 ppt 9.6%
- 33.0 to 34.0 ppt 41.3%
- 34.0 to 35.0 ppt 31.1%
- 35.0 to 36.0 ppt 7.4%
- 36.0 to 37.0 ppt 3.7%
- 37.0 to 38.0 ppt 1.2%
- 38.0 to 39.0 ppt 0%
- 39.0 to 40.0 ppt 0.4%
Temperature distribution
3,727 tanks. Median 78.0 °F. 4 readings outside 70.0 to 86.0 °F were dropped.
- 70.0 to 71.3 °F 0.1%
- 71.3 to 72.7 °F 0.2%
- 72.7 to 74.0 °F 0.3%
- 74.0 to 75.3 °F 2.2%
- 75.3 to 76.7 °F 7.5%
- 76.7 to 78.0 °F 36.6%
- 78.0 to 79.3 °F 42.4%
- 79.3 to 80.7 °F 7.8%
- 80.7 to 82.0 °F 1.3%
- 82.0 to 83.3 °F 1.2%
- 83.3 to 84.7 °F 0.3%
- 84.7 to 86.0 °F 0%
pH distribution
3,596 tanks. Median 8.00. 42 readings outside 7.40 to 8.80 were dropped.
- 7.40 to 7.52 1.1%
- 7.52 to 7.63 1.3%
- 7.63 to 7.75 2.1%
- 7.75 to 7.87 9.5%
- 7.87 to 7.98 6.5%
- 7.98 to 8.10 44.1%
- 8.10 to 8.22 21.9%
- 8.22 to 8.33 6%
- 8.33 to 8.45 5%
- 8.45 to 8.57 1.1%
- 8.57 to 8.68 1%
- 8.68 to 8.80 0.5%
Method
- Source: tank profiles and logs created by ReefBay app users. ReefBay is a reef aquarium app for iOS and Android.
- Excluded: deleted tanks, deleted accounts, ReefBay staff and test accounts.
- Counted: the latest reading per tank and parameter in the last 365 days, so frequent loggers do not dominate.
- Normalized: specific gravity is converted to ppt (1.026 SG = 35 ppt), all temperatures to one scale (shown in °F), ppb phosphate to ppm, meq/L alkalinity to dKH.
- Dropped: readings outside a plausible range for the parameter (for example alkalinity below 4 or above 16 dKH). These are almost always unit or typing errors. The dropped count is shown per parameter.
- Percentiles use the nearest-rank method. "Typical range" is the 25th to 75th percentile.
Cite this page
You may reuse these numbers and charts with a link to this page (CC BY 4.0). Suggested citation:
ReefBay. "What are normal water parameters in a real reef tank?" November 2026. https://reefbay.com/stats/water-parameters
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