I once was told to keep the intakes several to 6 inches off the gravel. The reason given was that if the filter leaked, it could not completely drain the tank, since below the intake tube there would be water for the fish till you got home and noticed the crisis.
Now I have never had a filter leak or drain the tank, ever. I doubt it would happen with a HOB
. But I could concievably see a hose going to a canister or sump sprining a leak, and gravity could drain the tank. Thus , I have left my intakes about 4 inches above the gravel.
I do agree with Brian, I want to draw water in to the filter from deep, and put it back at the top, and thereby get good circulation and turnover.
I think that where you put a single HOB
depends on how you want it. You will always have slower circulation spots, and spots where debris collects in the gravel. With filter outflow and intake placement you can effect where those spots are, but not prevent them.