May have already been mentioned (haven't read through the thread) but since sponge filters are primarily biological filtration, wouldn't adding any more be sort of pointless? Your BB, which I'm sure you know is what does the bio filtration, can only reach a certain population before the waste load produced by your fish simply isn't enough to sustain anymore. Actually, I'd say at 5 that size in a 55 gal you have probably already surpassed that point. (Then again, I have no way of knowing that and could be wrong.) Honestly I would probably take at least 3 out and run the two HOBs or a canister. With two 60g rated HOBs I cant see the need for more than 1 sponge filter in a tank that size.