I use to have a 55 gallon and went to a 125 gallon as well.If you already have fishes in the 55 gallon,take at least 25% of the water from the 55 gallon or if you didn't clean the tank today,you can take at least 30 to 40% of the water then,also what powerfilters or canister are you running?What I did to process the cycling quicker,was taking my penguin 350 that already established the good bacteria and left my canister running on the 55 gallon with my cichlids still.I put my cichlids in and not a single one is effective by it at all.I do recommend another heater though,even though you can run a powerhead to circulate the water,it still may not reach the other end at all.If that tank came with a undergravel filter,don't even bother running that hunk of junk 20+ year old technology for the fishes.No reason to have,if you can run a good canister and powerfilter.