Do you know of any that arent?
Eels? There are lots of type of eels. But the family of eels I know about are the Mastacembelidae.
The one that don't get up to 40" are Peacock Eel, Zig Zag Eel.
Then there the one that get up to 40" which are the Tire track eel and the fire eel. Oh and they grown a inch per month.
http://en.wikipedia.org/wiki/MastacembelusMastacembelus is a genus of many species of spiny eel fish from the family Mastacembelidae. Some species are relatively common in the aquarium trade such as the Fire eel (M. erythrotaenia) and Zig-zag eel, (M. armatus) (also called the Tire track eel although M. favus is properly given the common name of Tire track eel)