When frozen, fish build up an enzyme called Thiaminase over time. Thiaminase destroys Thiamine, an essential amino (aka Vitamin B). If a snake ingests too much Thiaminase, large amounts of Thiamine are broken down internally. This can lead to neurological problems and even death down the line. Fortunately, it is easily prevented by feeding live fish and supplementing with either vitamin B or non-fish food items. Also, I have heard fish in the carp family (goldfish, koi, etc.) contain Thiaminase when alive so avoid using them as feeders as often as possible.