Why would you want to limit potassium? Plants use about the same amount of potassium as nitrate, but there isn't a critical amount needed. And more than enough potassium has never, as far as I can remember, been suggested as a problem in an aquarium.

There isn't much commonality between salt water aquarium care and planted tank care, other than that both benefit from good cleanliness standards and good water circulation - again as far as I can remember from the reading I do.

Don't assume your nitrate test kit is giving you a good reading unless you calibrate it with water samples of known nitrate concentrations. Our test kits are not laboratory quality, and even if they were, laboratory testing is always preceeded by calibrations.