I thaw mine in a net, too - though I have tossed in a cube when I was in a hurry. For smallish fish they just ate the bits as they thawed and floated off, but larger fish will definitely try to chow down on the frozen wad - I can't imagine that would be good.
If you put it in a cup with tank water, warn any house mates - frozen fish food seems to totally gross out my family - go figure