Hi Fishhead,
18' is awfully deep, and I guess lots of things can happen in those depths that can affect the entire pond.
I don't have the answers to anything except catching crappie. We have a 3 year old pond, and catch crappie that are over 11". We find it's just as much fun as catching bass. In a pond that's smaller than 5 acres, it's pretty easy to control them if they should have a big spawn. My wife could probably catch every one of them if I would let her. If you don't have time to make hook and line population adjustments, they could become a problem.
