200' of 1/2" poly is only 8 sq-ft of panel size. The sun in your location, assuming no clouds, produces about 2400 BTU/sq-ft/day on a flat plane which would be about 19200 BTU/day. However, most solar panels are only about 80% efficient so that drops to about 15360 BTU/day. For 8500 gallons, that is 0.22F temperature rise. So I think your estimate is overly optimistic. The sun does not generate that much heat to raise it 1 deg/day.
In the heat of the day, the output of the sun is 310 BTU/sq-ft/hr or 41 BTU/min or 33 BTU/min out of the panel. At 3 GPM, that would be about a 1.3F rise from inlet to outlet. So your flow rate may be much smaller through the panels than you think it is.