the piranhas are actually very timid and shy. at least the red belly ones we have are. i've heard the black ones are pretty mean though. ours run away when you put your hand in the tank, but i'm still scared to do it! lol!!! check out the site we visit for info often, it may help with your eel.
http://www.monsterfishkeepers.comwe actually have lots of fish. 3 main tanks and my kids each have a tank as well. our 3 main tanks are 1 55 gallon for the 3 large (about 4.5 - 5") red belly piranhas and 1 red belly pacu (looks almost identical to the piranhas and is in the same family, but doesn't have teeth and is the same size or bigger than my large p's), 1 55 gallon i have setup for my fancy goldfish (got lots of really pretty fantails and various other goldfish with a japanese dojo fish, a couple of frogs, and a pleco and algae eater. the last tank i've just set up and begun cycling it for sea horses (only a 30 gallon). it has 1 chocolatechip starfish, 2 electric blue damsels, and a bunch of live rock with little hitch-hikers in it. as soon as its finished its cycling process, i'll be adding about 3 or 4 sea horses and maybe another fish or 2.
my kiddos have their own tanks in their rooms. one has a huge 55 gallon with a red-eared slider and a mississippi map turtle in it and a goldfish that was supposed to be food for the turtles, but they couldn't catch it and it grew humongous so she named it and has it for a pet as well. my other daughter just has a 3 gallon filtered beta tank with a male and female beta in it.