my g/f has piranha. They're nothing like hollywood makes them out to be. Yes, they will go after meat pretty well, but nothing spectacular.
We used to have a fair amount of floating water sprite in the tank, and would actually breed guppies in there WITH the piranha. Some killers...
They're also really skittish! Partly because they need a larger tank, but we keep them covered most of the time otherwise they freak out and go bouncing off the sides of the tank.
Otherwise, they are very INactive fish- just sit there and float most of the time.
Basically, get them if you like them as a fish, if you like the way they look, etc. But, getting them so you can have a cool killer-death-fish... No.
That, and they'll outgrow the 29g pretty quick...
Get a few moderate-sized cichlids or something. A Jack D would work in that tank for some time, and I hear they're pretty nasty!
Also, the fish store should be more than willing to take back the Jack after you've done all the 'growing out' for them. Just don't expect more than a fraction of what they will re-sell him for!
Oh, also think about a school of tiger-barbs in there. Like 8-12 or so. Those t-barbs will get very active, chasing one-another all across the tank! Just don't put any other middle-dewllers in unless they are equally fond of roughhousing.
