Drawdowns or low water can cause population imbalances as fish are concentrated and subject to more predation. Also a large % of your total water is gone - as T noted above and you have to deal with that and the O2 problems he also pointed out. IMO you need cover at all depths in a pond for reasons in addition to low water levels. I would start the aerator and add some cover to deeper areas. Also often well water is low in O2 so that should be addressed.