I agree with Rabbut on this. Using hot water is not a good idea, although there are many valid ways.
Heres a few that i can think of
1. Defrost the food on a plate (not in water) at room temperature. Pour off excess fluid, then add to tank.
2. Defrost in a cup of cold tap water. Once defrosted, strain using your fish tank net so the water is washed down the sink. Add the food to the tank from the net.
3. As per no. 2 but use water from the tank instead of from the cold tap.
Using hot water will not only deplete the mineral content, but also provide an ideal breeding ground for bacteria as the food will most likely get partially cooked.
Hallett,
the fish will almost undoubtedly eat some frozen food if you don't defrost it first. The food won't defrost instantly on contact with the tank water, but the fish will start eating it instantly.
Also, they don't deliberately put harmful pollutants into the frozen food, but there is always the risk that they might be there.