I wouldn't advise to chuck them in frozen as it can cause havoc with their digestive system. Imagine eating frozen sausages, Same thing.
I put the cube on a plate and let it defrost. While defrosting a liquid comes off it, as happens when anything defrosts. I pour this away as the fish can't use it in any way so all it does is pollutes the water. Once the liquid is poured away and the food is properly defrosted, feed away.
Alternatively, put the cube in a net and run the cold tap over it until it defrosts. This also flushes away the pollutant liquid i mentioned .
I wouldn't pour boiling water over them for the same reason i wouldn't defrost my sausages that way, you're creating an ideal breeding ground for germs. Letting it defrost naturally at room temperature or with a little help from the cold tap is the best options.