I've had this problem with my fish when they were young and it happened everytime i would run out of food and i would try something new. They would go on a hunger strike for over a week. The key is to offer a varied menu so the fish don't get sick. There is more value in the pellets than the frozen plus the price difference is staggering. So, tough it out and offer only the pelleted or flake food. Your fish will get very hungry and accept the food and after a week or so offer a small portion of frozen food as a treat.