ME Pharmaceutical grade Kalkwasser (Kalk) which is also known as Lime or Calcium Hydroxide. It is the only chemical that delivers both Calcium and Alkalinity together. It is very effective at maintaining not raising Ca and Kh.
In small systems with low consumption rates this may be the easiest way to supplement Ca and Kh. Kalk has also been shown to improve the performance of protein skimmers, as well as aid in the reduction of phosphates.
Add up to 2 tsp per gallon of RODI water, stir til dissolved and use as topoff water.