Work out exactly how many grams of dry chemical raise your alkalinity, calcium or magnesium — and see the arithmetic that produced the number, so you can check it against your own test kit.
The app fits a line through your last two weeks of tests, works out what the tank actually consumes, and turns that into a maintenance dose — with the same formula and the same safety limits you see here.
Nothing here is transcribed from a forum post. Every constant is derived from the chemical's molar mass, which means you can re-derive it yourself and catch us if we are wrong.
One dKH is 0.3572 meq/L. A mole of sodium bicarbonate supplies one equivalent of alkalinity; soda ash supplies two. So the mass needed per litre, in milligrams, is 0.3572 × molar mass ÷ equivalents.
Raising a parameter by 1 ppm means adding 1 mg of that ion per litre. A compound is only partly the ion you want, so you need 1 ÷ mass fraction mg of it. Calcium chloride dihydrate, for instance, is only 27.3% calcium by mass.
| Chemical | Formula | Raises 1 unit per litre with |
|---|
Sources genuinely disagree on how fast a parameter may move — alkalinity limits of 0.5, 1.0 and 1.4 dKH per day are all published. We take the conservative end, because the cost of being wrong is asymmetric: a slow correction is tedious, a fast one kills coral.
| Parameter | Safe daily change |
|---|---|
| Alkalinity | 1.0 dKH / day |
| Calcium | 20 ppm / day |
| Magnesium | 50 ppm / day |
If your gap is larger than one day's limit, the result is split across as many days as it takes, and you are told to test in between — because a split correction assumes the tank consumes nothing in the meantime, and it always does.
Concentrated calcium and carbonate exceed the solubility of calcium carbonate before they have diluted into the tank. They precipitate, clouding the water and removing both ions — so you end up lower on both than when you started. Separate containers, different high-flow areas, at least 20 minutes apart.
A calculator does not know your tank. It does not know that your test kit reads half a dKH high, that your sump holds less water than you think, or that the coral browning on the left side has nothing to do with alkalinity. Verify with your own test kit, move slowly, and treat every number here as a hypothesis about the tank rather than a setting.