on the whole cycle thing i would get bio spria if you have a location near you that sells it. i would almost double the ammont, like 3o gallon tank i would do a pack that treats 50 gallons, you cant over use it.
and when i got my piranha, they feed him live foods at the lps, so i had to stop that, so everyother day i put frozen krill in, left it for 10 mins, took it out. it littery took 2 weeks for him to eat it, so yes piranhas can last a long time without food.
also i run my lights around 7.5 hrs a day, they are on a timer i got a homedepto, its good the piranha gets used to the light going on it doesnt stress them out if the light comes on at the same time a day and they get used to it.