The way you use them is to buried them deep into the gravel. If you just put them on top, they will dissolve quickly. I had some issue using these a few years ago in my tank with pleco. The pleco would dig them up too quickly.
It's fine when they dissolve. The clay is very fine. If you ever look at these clay powder, they are very fine. You are most likely going to vaccum them if you ever use a gravel vac.
Hope this helps.