You have to be kind of organised to freeze your tetras right, I find. Remembering to put one in the freezer 3 hours before I want one very often doesn't happen, so I quite frequently end up with the concrete block. When that happens, I tear off the packet and whack it in the microwave for 30 seconds to a minute (take the packaging off, otherwise you get a bit of a fireworks display). So the bottom gets a bit warm but I figure it can't hurt much
Alternatively, and I discovered this by accident, take your rock solid tetra to work. By the time it gets to lunchtime, it's damned near perfect ice cream.