Well the recommended minimum in a shoal is 3 and they will do good in the 55 for a while but remember these fish can get bigger than 12" so if you plan on keeping them for life you will have to upgrade in the future (but not for a few years after they reach the 6"-7" mark their growth slows down and they only grow about an inch a year) for the temp you can keep it from high 70's - mid 80's and don't really worry about lighting unless you want a planted tank, reds don't really like light all that much. I would stay away from feeders because when your piranha eats a feeder it eats everything else that feeder has been with and been swimming in! If you really really want to feed them live breed the feeders yourself, but there is not that much nutrition in them. STAY AWAY FROM GOLDFISH!!! Try and get them to eat a mixed diet of pellets and white fleshed fish or frozen bloodworms, not only does this make your little toothy darlings happy and plump, it helps them grow faster and makes the red on their cute tummies become more dramatic! (If you want some help on how to get your piranhas onto pellets just pm me I can give you some tips!) Like it was said earlier, you are going to need a kick ass filter because piranhas are messy messy MESSY for a 55g tank i would get a filter that is meant for like a 60g I recommend weekly water changes of like 30%. Oh one last thing, you will want to have lots of hiding spots for the little guys, it calms them down and they are less skiddish and it will stress them out a lot less when they are getting used to their new home if they have places to hide and feel safe. If you have any other questions you can pm me or check out the stickies on the piranha board here on MFK!