I have the 65 model and dont have the sponge/return box on it at all! I dont know if they are different but mine works fine! Does yours have the little grey valve that connects to the top of the return neck with a piece of air tubing? Mine got clog and it gave me a hard time but after i cleaned that out havent had any problems!!
Good Luck! Im going to get a better skimmer for my tank!