Calcium hydroxide (slaked lime)
The chemical compound behind kalkwasser. Provides calcium and KH simultaneously raising pH and precipitating phosphates.
Detailed explanation
Calcium hydroxide Ca(OH)2 dissolves in cold water (max 1.5 g/l) creating a saturated solution that integrates calcium and carbonates in the natural ratio in which corals consume them.
It raises the pH and precipitates phosphates. It is the oldest and still valid integration method. Aquarium lime is purer than building lime.