There's tons of threads on RC on this topic. IMO... the short answer..... Eels create a very large bio load for a single fish... They will most of the time eventually eat your shrimp, snails, crabs and small fish.... because of their size they can knock over corals and move rocks that can cause lots of problems. I know many who keep them successfully in a reef but it is typically a large system and it is the exception not the rule. There's tons more to say on this topic which i'm sure others will but that's the quick story IMO. .... and.... different eels vary greatly from each other in the experience you will have so you will want to be more specific on the kind of eel you are interested in.