Thats why I mentioned above to limit the rock and any thats used should be elevated. We once talked about doing an sps tank with only a couple of pieces of rock, elevated above the plenum on piles {pipe}, and having most of the sps pieces attached to the glass and overflow. Nothing wrong with then having some extra rock in the sump, as long as it can be kept clean.
The limited use of rock in the tank, will also help relieve caclium demand with the coralline. The 4-5in. of gravel will provide more than enough biological filtration.
Of course this limits types of fish, that require lots of hiding places & rock, like many of the smaller fish & dwarf angels, etc. Once the sps grows, more hidey spots are there. I know if I was a fish, this would not be my favorite habitat, at least not at the start.
