I use a calcium reactor now. Yes I had to dose potassium when using comp123. You have to test your potassium often to see how fast it is depleted from your tank. Every tanks different since it's thought that many things use or pull potassium from the water including, skimmers, zeolites, animals and bacteria.
I recently have bumped my potassium up to a higher level and it's now staying more stable and I am not having to dose as often. This might change over time though, we will see.