I've been running my 29 gal this last 9-11 months with HOB filter and only thing in it is a coarse sponge and a pre-filter sponge on the intake. Just Saturday I ordered bio rings. Half the filter has bio rings, and I folded the sponge in half and tucked it in at the top. There is also a small sponge filter in the tank I added maybe 6 months ago. Doing fine so far. Did I need the bio rings? probably not. I figured I would give them a try though to add more surface area for bacteria since I am hoping to add some more fishes soon. Plus if ever I need to quickly cycle a new tank I can either pluck out half the bio rings or the sponge filter and borrow them to the new tank.