Thanks Ted. My guess was fish/nutrient load and possibly some sort of concentrated farm run-off/infusion ( black cattle waste). With the aerator running and the fish doing ok one evening and dead the next am indicates that something used up a lot of O2 real quickly over night. Unless there was a cold rain or spraying weeds or cloudy weather or the like there should not have been a plankton crash. Something caused a spike in BOD (or reduction of available O2) which could not be met even with aeration.