Thanks guys. I guess I'll siphon a bunch of the water out, and then drain the rest. There is an extra bulkhead in the bottom of the tank in the overflow box that I'm not using, so hopefully I can take that one out easier than dismantling the plumbed bulkheads.
Now that I've learned my lesson, what do people put over their overflows to keep this from happening? This fish is pretty small, like a little thicker than a pencil.