Break a piece of the rock with the sponge on it while underwater, put it in a cup with the tank water and put it in the sump if you like, it don't really matter. Make sure the sponge stays underwater at all time or it will die quick.
I agree with jay0705, also I wouldn't recommend a flame angel in a 29 gallon because if you are planning on having coral, it may nip on it. Try not to stock to many fish in a 29 gallon.