Are 2 of the AC-O medium sponge filters sufficient for a 55 gallon?

[JenTheDroid]

Or do I need the large? Just asking since the large ones do take a larger amount of space. I’m worried they will be a little snug. 

[DaveO]

The two medium sponge filters should give you plenty of biological filtration. I used the same for my 40b without issue. It will not provide good mechanical filtration however. Depending on your stocking, you might need to add a hob or canister filter. I added a Fluval 207 canister filter and removed one of the sponge filters because the shrimp in my tank kick up debris quite a bit. Now the tank runs clear.