having a thin substrate is no replacement for a canister filter/ bio filter. get no more results than you would with a thick substrate. water flow is only going to hit whats top, thick or thin, on my 180 I have 6" crushed gravel on a reversed UG system, duel bottom drilled, running 2 pumps. pushing 2400 gph with 9" foam filters on the intakes. this the only time I can say your gravel "may' be good for bio filtration.