My dad used to have a piranha and he would always tell me stories like when he was low on money, he would feed it bologna but it wouldn't eat if he was watching. He would have to sneak around a corner to get to see it. He only had one piranha and kept it in a 55 gallon. As a lot of other posters have stated, they are probably just stressed out from the travel. I would recommend testing your water. As long as there isn't anything abnormal, I wouldn't worry too much.
Like I said before, water is perfect.
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