Dale,

Every pond or lake is different, and there is often more than one contributing factor, and from the sounds of it you might have a few things working against you. Have all your fish kills been after a major storm? Or are there any common conditions between your fish kills?

If the phosphorous runoff is excessive you may be fighting a very difficult battle. You may be having huge swings in oxygen demand. Is there any way to limit the runoff from the fields?

The only real way to know what is going on is it get yourself a DO meter and start monitoring your water. It sounds like you have O2 issues, but getting to the root of them is going to be critical. The DO readings should help fill out the picture and let you know for sure what is happening and when.

Regarding aeration, it would be helpful if you could provide an image from Google maps so we could see just how irregular it is. You have some very nice depth at 30 feet, and a good bottom diffuser at that depth will move a lot of water.