Don't buy pre-frozen fish. There are things they put in in for 'human consumption' that is not safe for or reptilian friends, I believe it's called 'trisodium phosphat' something like that, but either way it's bad. Buy only Fresh Fish.

I a while back went to the bait shop n they were out of 'dew worms' and had a container with what were labeled as 'red wigglers' But I opened it n looked, they were mis-labeled, I took one n squeezed it n pinched it with my nails, n there were no secretions so I used them, and they were fine. Just have to be VERY careful to be sure which they were. If in doubt, don't feed em.