reef code a and b is a 2 part alk and calcium additive. Kalk is a powder that you dissolve in ro water and have a drip set up. In my experience kalk is more stable since it is one thing being added at a constant drip and amount, it also makes the corals grow faster but.... what i know is that with the calcium there is better coloration in the corals since the grow a little slower. if you have room to set a 5 gal drip up id say go for it, its easier and you only have to refill it once a week