Echo Catfish Are Cool's post...
They NEED:
Sand
Heavy plant cover
Things to hide behind
Dont use heavy rocks unless they are sat on the tank bottom rather than the sand, they can get trapped under them. Dont have ANY openings inthe lid, you may need to cover ext filter pipes with a bit of old tights or stocking, dismantling the filter to extract an eel is Not Fun.
Eels use the substrate to hide in when stressed and they dive in there really fast, gravel and anything sharp will cut them and healing a cut eel is really hard.
If you dont have lots of plant cover then your eel will spend all its time either in the substrate, or dashing about madly, and basically... it wont live very long.
When you buy one, make sure its nice and fat, and ask what its been eating. Be prepared to feed a lot of live bloodworm at first as this is often the only thing they will eat at first.
For your tank size, you could have a zebra eel, a peacock or striped peacock eel, a burmese black spotted eel...
I have had all these, the zebra eels are doing well, the black spotted is doing VERY well.... sadly neither peacocks made it and thats purely down to feeding, neither would feed, either because the tank was too busy or they just didnt like bloodworm and never had.
Not feeding is very common with spiney eels, offer live bloodworm, offer it after lights out.
Your tank may well be too busy for a shy eel, of all my eels (barring the tyretrack type who lives in solitary due to being a Right Nasty B*stard) the Black Spotted is the best eater and he now competes well in a tank with similar tank mates to yours (i have lots of big angels, rainbows, etc)... this has taken a lot of time and dedicated hand feeding. He will now compete with the other fish but thats taken the better part of a year.
If your tank is very busy (and id think yours is), go for the biggest of whatever eel you get, going for a 5 inch peacock eel is going to be like paying to watch it die in your tank, shell out the money for a nearly full size one and you will have a much happier fish.
Zebra eels are the smallest adult size you can get, so they will pretty much always eat bloodworm, you can try getting them onto brine shrimp and daphnia but not all will take to it (one of mine will, one wont).
The two peacock types (actually totally different eels just a similar common name) and the black spotted eel will get big enough to take small prawns, my two bigger eels take prawns, bits of squid, mussels, cockles etc, although some of it needs to be cut up into strips.