Go saltwater and get an eel that stays at or below 4' & is on the thinner side (e.g. snowflake eel, etc.).
Gulper cats definitely get large (but not too large for your tank).... but I have never heard of them referred to as "active". Not particularly colorful either.... but they're cute in a fat/disgusting kinda way. I've been looking everywhere for one for my 90 gallon. I just want to hand feed a fish again like I used to with my Oscar.
< very good single fish, more personality than any other fish I've kept and ivehad a few