If I remember right you have a spring feeding your pond. How far does the water flow from first surfacing untill it enters your pond?
Most springs are devoid of oxygen. If sufficient grade is available you could possibly aerate the spring water before it enters your pond. Cecil Baird feeds his trout pond with well water which he aerates.
With only 5 ft depth you shouldn't have stratification & the low DO associated with it.

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I drop my approximately 52 degree well water fresh out of the well into a series of five gallon buckets filled with plastic media to break it up adding oxygen (well water has zilch for oxygen)This also blows off gases like nitrogen and sulfides and carbon dioxide if you have any.
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