Big debate on gravel
Over in the pond forums I used to frequent, there was a long and rough debate about gravel in ponds. Essentially, one must clean it frequently, or eventually there will be an erruption of some sort of nasty "something" that will kill fish. Rather like having an aquarium that is never cleaned, or a UGF that is never cleaned, the toxins build up. The fish will be fine for quite awhile, then things start happening, finally when the gravel is cleaned, it smells horrid.
So, invest in a pool cleaning device of some sort, there are some that attach to a hose and work like a pool vac. Or get a large aquarium style gravel vac and clean it during water changes.
Personally, I try to keep my pond clear of gravel, but the fish dig in the plants so much that there is gravel all over anyhow. Oh, I need to get out there and do the fall clean up. Bleah.