I thought I would add that the stressor more likely to be causing these problems is low DO especially at night. You did not mention if you aerate. Some kind of surface aeration might help improve condition but it comes with this warning. With this level of bloom and the already existing stress ... any mixing of water not penetrated by light has an elevated risk of exacerbating DO drawdown. Also, the byproducts of anoxic biodegradation are toxic to fish. Turning the pond over would probably have devastating consequence. Paddlewheel aeration is very efficient and is least likely to cause mixing but this really depends on the power of the aerator. In other words, too much power and the water may still mix too much. I know some members have used boat motors and water pumps as aerators during events like this. I think I would prefer to do such with a water pump in the shallow end of the pond further away from the deepest portions where anoxic volume will be greater. if done correctly only the surface of the water is disturbed and a current develops like a mushroom cloud where the disturbed water circles back on both sides of the stream to reenter the disturbed portion (like two eddies).

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers