I am in this exact same situation as you and just made a thread about it last night. Same thing: upgraded to a 20 gallon, got excited, bought a lot of fish, used the 10 gallons of cycled water from my other tank, didn't cycle the tank properly, got freaked out and came to the Monster Fish forum for help, lol. Hilarious.
What I did: I bought a AquaClear 50 at the suggestion of some people here. Put that on the tank along with my old filter from the 10 gallon for bacteria. Got my fingers crossed. Hoping it works. Good luck to you too!