well, I get my myssis in frozen cube form also, but my cubes are very small. What I do, is I rinse the cube in RO
water first, then set it in a spoon to defrost. I then soak it in either garlic or selcon (I alternate). After about five minutes or so, I crush it up with a fork if it's still hard.
Then, feed it to the fish...they love it.
The only other thing is that if your cube is huge, you could just cut part of it off with a sharp knife and basically just do the same thing.