Be careful with K+ dosing, as unlike a lot of trace elements, overdosing of K+ can have quick detrimental effects. And while both KZ and FM make K+ test kits, they are notorious for being hard to read...
If truly concerned about potassium levels you're best bet is to use a salt mix known to have decent (or NSW) levels, and preform regular (weekly) WC's...
Also with Zeo sytems the current thinking is that the bacteria dosed consume K+ along with the nutrients in our tanks, causing possible lower K+ levels. But as Genetics said, none of this has any scientific backing, it's all anecdotal...