Please help!!

I upgraded to a 55 gallon tank due to my 10 gallon being very over stocked. I had 15 tropical fish with 3 snails in my 10 gallon. I set up the 55 gallon and used the new tank water conditioner and got the water temp to around 72 degrees, ran for about 4 hours before any of the next fish steps happened. I also have 2 60 gallon filters in the tank. I transferred the fish by putting them in a bag with their old water and let them float for about 20 min. Went to bed 2 hours later and they were all good. Woke up 6 hours later and all of them where dead. none floating on top but all on the bottom or stuck to the filter pump. Please help me. I removed them and have the tank running all day today.