Sounds like you might be putting too much sinking food in your tank.
If it is dissolving and laying on the bottom uneaten, and then floating around when kicked up, you might try feeding less and see how that works...
Sand is way better than gravel. I only have an emperor 400 and a cascade 1000 (total of probably 600gph actual flow) in a 75 with no powerheads and my sand is kept pretty clean.
