I tend to let it defrost before I put it in. I usually put a cube in a pot and then submerge that in some warm water to help it defrost more quickly. Generally sinks to the bottom but usually one or two stay floating at the top.
Well the way I tend to do it is that I take water from the tank and put it in a plastic cup and then I let the Brine Shrimp melt. Usually it takes about 5 minutes or so for it to completely defrost. Then I feed the fish bit by bit so I do not overfeed. This is what I have always done. But I use the brand Hikari but some other brands have a more effective method of freezing the food. But the method I use I have found to be the most effective. Hope this was helpful