sry about the pic quality i saved this piranha from a lady who was gonna flush it she told me it was 6 years old and it was in a 20 gallon tank and i am talking about the big one i know the other ones are RBP
not 100% positive because the pics are blurry as hell, but that would be my guess. It's easy to confirm S. sanchezi. The scutes will be more irregular on a sanchezi than on other serras.
I hope if you do take it you rehome it to a larger tank. You might want to sit on the floor in front of the tank and wait a bit before snapping a pic, I have to wait for mine to forget im sitting there to get the good pics.