KALKWASSER

dKH/day = 4.26 L × 1.500 g/L × 75.58 dKH·L/g ÷ 284 L = 1.701

Calcium hydroxide Ca(OH)₂ has a molar mass of 74.092 g/mol and supplies two equivalents of alkalinity per mole along with one calcium. One gram in one litre therefore gives 75.58 dKH and 540.9 ppm calcium.

Those two numbers are in the ratio 7.16 ppm calcium per dKH, which is exactly the ratio corals pull them out at. Kalkwasser cannot push calcium and alkalinity out of balance, which is its main advantage over two part dosing.

Saturation is about 1.5 g per litre near reef temperature. Calcium hydroxide is unusual in that it gets less soluble as water warms, so saturated limewater made with hot water holds less than with cold. Saturated kalkwasser runs roughly 810 ppm calcium and 40 meq/L alkalinity, matching the figures Randy Holmes-Farley publishes.

The old rule of two teaspoons per US gallon works out to about 1.3 g per litre, close enough to saturation that the excess settles out. Adding more lime than saturation does nothing except leave sludge in the reservoir.

Evaporation is the hard ceiling. Limewater goes in through the top off line, so the tank can only take as much lime as it evaporates water. A tank with heavy demand and a tight lid will not keep up no matter how it is mixed.

The pH effect is real and useful. Saturated limewater sits near pH 12.4, and dosing it raises tank pH, which is why kalkwasser is a standard answer to a tank stuck at 7.8. It is also why it must go in slowly, ideally overnight when pH is lowest, and never as a fast slug.

Start at a quarter strength and raise it over weeks while watching pH. Going straight to saturation on a system that has never had it swings pH far enough to matter.

Let the reservoir settle and draw from above the sludge. The undissolved lime at the bottom is calcium carbonate formed by reacting with air, and it does nothing in the tank.

Sources: Holmes-Farley, DIY two part (Reefkeeping 2006) · Holmes-Farley, Low pH causes and cures