I think he was lying
It's possible but not much fun, I'd say the Koi would be extremely stressed if it is true.
Incidentally, my new tank has just finished cycling and I'm not on the hunt for some juvenille Red Belly Piranha. I've been building up to this for a year, getting bits and pieces, research and setting up the tank.
Does anyone know where is the best place to get RBP's? I've been to Newlands Cross and they have some sometimes, Aquatic Village have some larger ones last time I was there an my LFS said they can get them in.
Any recommendations ?
Cheers,
Matt