What is the source of the water flowing into your geothermal system - a Well? Does your system have a heat recovery unit on it? I would say the main consideration would be to splash the water across rip rap or filter media (like a 5 gallon bucket filled with wiffle balls) to oxygenate the water. When I looked at oxygenating well water (which contains no oxygen) into our ponds last year, I explored venturis, but settled on rip rap.
On a much grander scale, I have Trout fished the waters flowing out of Lake Taupo in New Zealand where they are big into geothermal energy production. The temperature of the river is raised by approximately 15 degrees F from the discharge. Needless to say the Trout are plentiful in the confluences of the colder tributaries.
I, too, am building a pond in central Oregon, where I now live, and doing a closed loop (in ground) Geothermal system. As I gather more information, I will post pictures and tales of my experience.
