Hi there! Welcome to the hobby and to AA!
First things first, what are your water parameters like? Are you using a used filter from your friend and was it basically kept wet in a tank with fish throughout the transfer from your friend or was it ever in storage or washed in tap water? I'm trying to determine if the beneficial bacteria is still present in the filter. It sounds to me like your fish may have died from "new tank syndrome", ie an uncycled tank. If this isn't the case, ignore the following:P
Cycling takes time, usually around a month. Basically, colonies of bacteria need to form on the walls of the tank, the gravel and the filter, that can break down toxic ammonia from fish excrement into (still toxic) nitrites and finally into less toxic nitrates. You should only add something in the order of 2 fish a week and only after the tank is cycled. It's not a total disaster that you didn't, it just means you have to work hard for the next few weeks to keep your fish alive.
If you don't already have one, I'd recommend you go buy a good test kit like the Master Test Kit from Aquatic Pharmaceuticals and do a [acronym:385a1e1f18="Partial water change"]PWC[/acronym:385a1e1f18] everyday to keep ammonia <1ppm and nitrites <0.25ppm. The better option that will save you time but cost you some money up front is to find yourself some Biospira tomorrow and use enough of it to treat your 55G. It will cycle your tank overnight. In the meantime, you will still have to do PWCs. (I'm assuming you know about using dechlorinator for water treatment so I won't go into it here, but feel free to ask!)
I would remove the under gravel filters you're using as they could get troublesome in the long run. Your other filter is probably a Hang-On-Back ([acronym:385a1e1f18="Hang On Back"]HOB[/acronym:385a1e1f18]) power filter. Depending on the flow rate, you might need another to supplement it. Do you have a make and model for the filter?
Good luck! We're here to help you through this, so fire away at the questions and keep us posted!