People use Potassium Chloride to provide Potassium in aquariums. How it dissolves may depend on its purity. I would not add a pound block of it into a filter and hope to let it leech into the water column. It is used as a table salt substitute, personally never used but I imagine it also acts like table salt in water. Be wary about what impurities there are in your garden Potash, might be harmful things in there. There probably are different grades of it, like garden, food, pharmaceutical. Probably is a limit to the K for safety but you are also putting in Cl into the water. If the stuff you have is pure enough then you can use it for dosing, but not that style of coral buffering you have done in the past.