He is buy him self , I would not try to feed anything for the first 14 days Explicitly if he is wild ... Keep Perfect water quality all around .
Make sure lighting and all other factors are repetitive , Piranha like the same thing day in day out make them less shy IMO .
Then add just a smidget of food on the 14th day (5-20 days would be adiquite) I would go with salmon or silver side if you want a pellet I think it would be a must for Hikari Massivore its like crack to Piranha thay cant resist