the key are "PATIENT" my friend.
It's hard for piranha to change diet with in a day or two. Here is I would do.
From now on this week try to feed him raw shrimp for the whole week with small portion. After 30min if your piranha doesn't eat, take the food out. Try again the next day.
Next week try silver side or fish fillets. Again with small portion feed him your normal time, after 30 min take the food out.
Keep feeding them variety different food ever week, do worry to much about piranha nothing. They can go with out food for days, weeks, some time months. Just think about in the wild amazon. they don't get fed every day.
I would add power head, dimmed the light and make them very comfortable in the home first. IT will take time but it work.
good luck.