Well, I know of a spring in Austin that flows in at around 60-65 degrees year round due to the depth of where the water is coming from. If your water coming out of the sump is in the mid 50's, it is very likely that the temperature may stay around that level year round. Now I know a half acre isn't exactly small, but having a constant source of water coming in from the mid 50's would do a great job of keeping the temperature down even during July and August. This opens up all kinds of possibilities for your stocking strategy. I am interested to see what you decide.

Water dries, rocks crumble, and trees die. The only thing that is eternal is the reputation we leave behind.
- Ancient Viking Proverb