If well water is the problem you can add oxygen to it by making it spray into the air before it goes into the pond. Coming directly out of the well it is mostly devoid of oxygen. Assuming your well water is good water.

Don't know how much water you are adding, but too much water exchange can be stressful. I doubt if that is your problem but just another possibility.

Some sort of surface agitation will help oxygenate the water.

John

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