Alkalinity, nitrate, and phosphate units side by side. Two of these trip up almost everyone at least once.
| Unit | Value | Typical kit |
|---|---|---|
| dKH | 8.00 | Salifert, Red Sea, API |
| meq/L | 2.857 | Seachem, ESV |
| ppm CaCO₃ | 143.0 | Hanna HI772, HI755, API |
| German degrees, °dH | 8.00 | Same as dKH |
Natural seawater runs about 7 dKH. Most reef tanks are held between 8 and 9. Tanks running low nutrients and heavy light sometimes go lower on purpose, closer to 7.
1 meq/L = 2.804 dKH = 50.043 ppm CaCO₃, rounded to 2.8 in this hobby
Alkalinity: one milliequivalent per litre is half a millimole of CaCO₃, which is 50.043 mg/L. The strict definition of one dKH is 17.848 mg/L CaCO₃, which makes a meq/L equal to 2.804 dKH. The hobby rounds that to 2.8 and so does this site.
Rounding to 2.8 has one consequence worth stating. To keep all three units round-tripping exactly, this converter treats 1 dKH as 50.043 ÷ 2.8 = 17.873 ppm rather than the definitional 17.848. The 0.14 percent difference is far below what any test kit resolves, and the alternative is a converter that disagrees with itself.
Nitrate: the ion NO₃ weighs 62.004 g/mol and the nitrogen atom in it weighs 14.007. The ratio is 4.4267. A kit that reports nitrogen reads 4.4 times lower than one reporting the whole ion, on identical water.
Phosphate: PO₄ weighs 94.970 and phosphorus weighs 30.974, a ratio of 3.0661. Hanna ultra low range reports phosphorus in parts per billion, which is three unit conversions away from the phosphate number most people quote.
Neither conversion is a matter of opinion or kit quality. They are fixed by atomic mass, and the only mistake possible is reading the wrong scale.
Volume: the US gallon is 231 in³ exactly, or 3.785411784 L. The imperial gallon is 4.54609 L exactly. Recipes and dose rates from UK sources mean the second one.
Sources: Dieck, Reef Chemistry Calculator · UNESCO EOS-80 / Millero & Poisson 1981