Sorry n3ont3tra, but a 55 for most types of eels is not ideal when its a reef tank and as is, a 55 for a SFE for it life span is also not ideal. Most of what one can do within a 55 when a reef tank, the only ideal type of eels at most are the dwarf morays. And even yet more of an idealist in that be that its best for when keeping any dwarf eels long term in larger reef tanks then smaller tanks due mostly to the methods in which you need to feed them.
As for not keeping any eel and only offering others things such as they produce lots of waste and when so many are having success no less. The idea there be is the type of skimmer and system setup one has in which its not simply good to tell one they shouldn't at all when your facts are based on your own opinion.
So again, for the most part of the idea of some eel in a 55, your looking at problems due to the first reason as your tank isn't that large and a 55 reef tank is too small for most eel other then dwarf morays. So I like to make it clear that even if a FO tank this 55, a SFE cannot stay in this tank forever.
Also the Caribbean Tang will become to large for this tank that it needs a tank twice the size>
Even if its the blue tang, it too requires a larger tank then a 55>
If a FO tank, there are many eels that grow 2' and less and you not be able to keep many other fish in this tank.