What I do: first layer of filtration (mechanical) is the fluval edge prefilter. This prefilter sponge has smaller holes than the AC block sponge. Having that said you have all the room in the HOB with anything you want to fill. For me I use purigen 100ml bag (bottom layer) then I use seachem matrix for the rest of the space.
The only downside to this is the prefilter sponge need to be cleaned every week.
And unfortunately I don't think HOB can remove nitrates because the whole HOB is heavily oxygenated. The only filter I can think that can remove nitrate is canister because of the vacuum inside and the lack of oxygen in the canister.