Wouldn't need to fully empty the tank, half way or a bit more would suffice. There are two ways to fix this.
The right way: Get a new frame and replace the old cracked one. ( See youtube videos on how to do this ).
The bandage way: Drain some water and use a bit of epoxy to mend the crack ( JB weld works good for this ). You will need to use a clamp to hold the frame tight until the epoxy cures.