Once again Foster and Smith / Live Aquaria screw something up. I wish they would shut their doors until they could figure out what the heck they are selling. NO WAY you should sell something when you don't know what it is.
Fiddlers get kind of big (4 feet or more). No way those will do well in a 500 gallon. Shoot, we held our pups in more space than that. They are active and need open space to move around. There are pics of some I've had in the "Sharks" album in my sig.