Interesting idea, you would probably get some nitrifying bacteria in the Biohome, but maybe not much denitrification since you would have oxygenated water coming up through it and denitrifying bacteria need an anaerobic environment. I don't see any reason why it wouldn't work, although if you have smaller size gravel on top of larger size Biohome, over time the gravel will sift down and the Biohome will end up on top. You would probably have to put something like fiberglass window screen between the layers to keep the gravel from sifting down.