First off Join this site, this is a site I belong too, they are all piranha experts. http://www.piranha-fury.com/pfury/index.php
Secondly piranhas are tropical fish obviously so like all tropical fish, they need a tropical setting. You could kill them by changing around the Temperature like that, they need a Constant Temperature of 78-81 degrees. If you can't provide this most basic of care to them, take them back where you got them from.
They are still stressing from the move and from the lack of the right water Temperature. I wold wait another day at least before you try to feed them again. At that size you only need to feed them once every other day, I would still feed them daily though once they get acclimated.
