If your going to have small fish, like gobies, then it's not reef safe as there is a chance it may eat them specially if they're small. Also it will probably go after inverts. Alot really depends on the size as really small one probably wouldn't be a problem. The biggest problem will be that eels are messy feeder and produce alot of waste, some will even throw-up their food. So you need to worry about nitrates going up.