Anytime there is a DO problem and fish are at the surface gasping for air you have an emergency situation. You need to create an O2 sanctuary asap. Any type of tool that will churn up the surface water should be used. You can move diffusors shallow 2 ft , use a boat motor/prop to churn water , or other method. You can figure out temp and causal matters later but get some DO into the surface waters. Fish can sense and move toward areas with improved DO status. Breathing comes first !
See this
https://srac.tamu.edu/serveFactSheet/292 
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