well this issue comes up alot with the turtle people. The biggest problem is thiamine (vitimin B-1) deficiency. Freezing can lead to destruction of thiamine in some fishes. In the turtle world, we usually just reccomend adding some more pelleted (vtimin enriched) foods to the deit of any fish eaters which will be fed thawed (previously frozen ) fish. with snakes, i supoose its different....though perhaps you could put some pellets into the frozen fish bodies...i do it with a tweezer to get vitimins into turtles whcih won't eat anything except fish. On the plus side, freezing as you snake guys know does kill some (not all parasites).