ewest,

There are many good maps available of ground temperatures. Basically, in most places the heat flow flux from the center of the earth is so small that once you get deeper than 10', the ground temperature is a very close approximation of the integral of the air temperatures over the course of the "average" year.

Below is a link from the heat pump guys. Their calculations require a fairly accurate estimate of your groundwater temperature.

https://assets.supply.com/ul_pdfs/284997_GroundWaterTemperatureMap.pdf

The ground temperature sets the lower limit for his "average" water temperature below the pond's thermocline. (Obviously, he will start with colder bottom water coming out of winter.)

After that, the calculations get very complicated very quickly. However, circulating water is very efficient at equalizing its temperature. Once he starts aerating from below his thermocline, he will lose his pool of cool water pretty quickly.

I don't see any possible way he could have an entire pond with 65 degree water with prolonged exposure to 90 degree air during the day and 70 degree air at night. (If that is his desire.)

The only exception would be having a deep, still quarry right beside his 2 acre pond. You could then pull cold bottom water for the duration of the summer and make it work.

I have been trout fishing around Atlanta, Georgia. Some of the spillways at the huge reservoirs have bottom drains that provide the only release water during periods with low lake levels. There are trout farms that exist just below the spillway outlets and can pull in this 60 degree water. That is the only way to make the physics work if I am understanding the email question correctly.

Rod