I'm not sure what kind of fish you have, but an easy way to make a FW look like a SW is to do an African cichlid tank. I have a mbuna tank (type of African cichlids) and it is often mistaken as a saltwater tank by people. It is 58 gallons and I stacked 103 lbs of lace rock up the back. It gives it the live rock "feel" without the coral. Also the cichlids are so bright that many people that are not that familiar with fish think they must be SW. Anyway, that is my two cents. You must do your research with cichlids due to compatibility issues and aggression, but if you research them alot, you can end up with something really nice. I have kept fish for a while now and nothing even comes close to cichlids. I will probably never raise anything else ever.