Thusly Marco Schwarz Spake Unto All:

You make me think about keeping one of our European eels.

Having tried, I'd advice against that. They'll eat any other fish in
the tank which they suspect will fit in their mouth, they'll rearrange
anthing in the tank that's not physically bolted down (not kidding -
my 50cm eel pushed around rocks the size of footballs; every morning
the tank was completely rearranged), it's an escape artist, and yes,
it's nocturnal.

There's plenty of interesting european fishes to keep in aquaria, but
eel isn't really one of them. IME, IMO, YMMV etc.