Clown Loaches grow to 12", however very slowly, but they do get big eventually. They are also best kept in groups of at least three. Many people keep CL's with cichlids, however the cichlids will way out grow them and in a 55g once there is such a difference in size it may not work out well for the CL's. They are beautiful fish, but in my opinion, if you want to keep them long term I'd wait until you have a bigger tank for them... the 75g would be ok along with the Oscar, depending on temperament of the Oscar and the size of your CL's. RD in a 55g with them, prolly not so much.
