I have acquired a few tanks and HOB filters, and I got to thinking about ways to improve the filtration efficiency, but I want some opinions. It seems to me the problem with HOB's is that you need to buy the new inserts for each specific filter, and they are also quite small and don't seem like they'd have that much area for bio filtration. Also, I don't think you really need to ALWAYS have carbon in there. What I am thinking of doing is buying some filter sponge and cutting it to completely fill the area between the impeller and the outlet waterfall. It seems like you could fit a whole lot more media in there and house much more bacteria. Maybe do it in two sheets so that if you want to have some carbon in there you could sandwich it between the foam in a small bag. Does anybody see a problem with doing this, other than maybe lowering the flow with more media? What have you done/seen done to improve the HOB?