Frozen fish can be deep fried but you don't want to put frozen fish into hot oil in the deep fryer because it could create a splash back of the hot oil and burn you or burn you and catch your house on fire.
But it's best to thaw the frozen fish out some first so that the deep fryer cooks the fish more evenly and you don't have to worry about burning the outside of the fish but have raw fish in the middle of it.
I always thaw my fish out first and even thaw chicken before I deep fry the fish or chicken as it cooks much better and more evenly.
So if you want the best fish from the deep fryer you should thaw it out first and then deep fry it for the best results.