I have an overcrowded tank problem--tropical fish?

I have about 16 fish in a 20 gallon tank, all about 2 inches or less in length. It'snot my fault because I got fish from PetSmart and they were not labeled male and female. So I bought two fish (not sure what type, but they were yellow and about 3 inches in adult size; I know they were some type of molly for sure, though) and they bred with some platy's that I had and ever since they have been having baby fish. So now I have mixed breed fish that the pet store surely will not purchase from me and my tank is really overcrowded. I really don't know what to do. It seems like I'm out of options. I can't buy another tank because I have nowhere to put it. I don't know anyone who needs any fish. The only thing I can think of is just killing them but I want that to be a last resort unless there's some way to humanely kill them. Last time I had to kill a fish was because it was too aggressive, much more so than other of the same species in my tank. All I did was put in in unheated, unfiltered tap water and I felt like it was cruel to do that. What should I do about my situation?