Here's what I see happening all too often in the scenario in my head-Family won fish and is driving home, kids/mom/dad suggest that the fish should be freed, and fish is immediatly let go at nearest road crossing. This might not be a big deal if just on person did it, but get a family with multiple fish, or carloads of people with the same idea, and you have a new invader on your hands. For this reason, I think you should lean towards warm water fish and give them away in the warmer months. That way, when the first cold snap hits, that tropical fish that miraculously survived the summer in a little creek will die and not live on to reproduce with the other dumped fish. There are still some creeks and rivers without wild goldfish populations and they should be kept that way. White clouds and danios are also possible invaders. I don't like the idea of fish as prizes in general, but if you're going to give out "throw away fish," consider ones that once thrown away, will not establish populations in native waters.
