This Old Spouse
TOTM Winner May 2013
Why take any chances? Just defrost in a container of tank water and pour it in as Noahs Ark6 suggested. I would think you're taking the chance of injuring the fish in some way by putting a frozen substance in the water. Whether it's from cold pockets or from a fish pecking at a frozen cube, they aren't equipped for it, so don't do it.