Why wont my 3 (4-5") Red Bellied Piranhas eat?

ive had them for 3 days now they have been in a nice 60 gallon tank for about 2 days, im new to piranhas but i dont want to mess this up. i have tried feeding them fresh frozen tuna but they are not interested, i have tried feeding them bloodworms they nibbled a bit but it doesnt look like they are very interested. they seem to take cover alot, they swim around when im laying in bed but when i stand up they all go and hide, why is this? how can i make my piranhas mean and aggressive, they have a bunch of little fish in the tank that came with the tank i bought, could this be making them unhappy? i was hoping they would eat these fish, they are all about 1/2 to about an inch there are about 6 of them, but the piranhas dont pay any attention to them at all. i got piranhas cuz they are beautiful and i wanted a really aggresive mean fish, i want my piranhas to rip to shreds anything i throw in my tank, how do i get them to do this? Also ive heard it could be because of filtration, but there is no way, because i have a 404 fluval canister filter, a whisper 50 gallon hang on back, an aqua tech twin waterfall not sure how many gallons for, and an undergravel filter with 3 hookups 2 are to nice air pumps and the middle one is connected to my 50 gallon whisper. and i also have a 30 gallon powerhead. so i beleive i have good filtration if not great, but please help me because all i want are happy aggresive piranhas.