Try feeding tilapia or goldies alfalfa pellets. I just got a 50# bag from the local feed store for $15. Besides being 19% protein, it has lots of other nutrients. It is 2.4% potassium.

Don't use rabbit pellets as it is loaded with copper and zinc.

Potassium in food is not the raw metal and won't explode. It might be cool if bananas exploded, but I've never seen it.

Blood meal is also a good source and has iron too. Add to the beds and use sparingly.