It sounds like a depleted oxygen fish kill to me. I have had two, one in winter where the snow was so deep that I didn't remove some from the ice to allow sunshine to get to the plants to create oxygen. The other time was a really bad summer drought where the fish were gulping air at the surface and bluegills were laying flat near the shore trying to get air.
My pond is deeper then yours but I still consider it a shallow pond that needs plants for overall health including oxygen from plants. You killed off your plants with chemicals and the few plants that were left were taken out by your grass carp. Now to solve that problem you want to take out your grass carp. Your aeration couldn't make up the difference for the co2 needed. In my opinion you need to pick one course of management and stay with it.
