Sorry for the delay in response, its been a while since I've been able to get on a computer, and it won't let me post from my phone. Thank you very much for your responses!
Roy - I have been considering using copper pipe, and it would be great to have it hooked up with a pump controlled by a thermostat. Although I am trying to stay away from using too much power, because I am hoping to get the greenhouse on alternative power. I like the idea of gravity flow, but don't know too much about it. I have read a few posts that talked about using gravity flow, but there was not much details on setting something up.
Andor- Thanks for the link. I really like Robs set up, but wished the video had more details on how he has the water heater part of the rocket stove set up. The pellet stove part of the heater is brilliant! I agree that I may be asking too much from the system. The reason I was planning to do a 6" system was the csa for the opening in the cinder blocks, as well 6" ducts and stove pipe has been much easier to find. Although I'm not sure if the 6" system can handle what I want it to do. Part of the desperation was that I was hoping the get the pond and Rocket mass heater in place while we were still in the worst of winter to see what kind of temperatures I could maintain in both the greenhouse and the pond. But alas the time has passed and spring is upon us. I'm think I'm going to try the 6" system, and connect the ducts through the cinder block wall of the pond, and if it doesn't work out so well try the copper pipe.
Beth - I agree completely about the investment of time and money, I am both on a very tight budget, and I have two small children, so the majority of work has to be done during nap time.
I know it would be far easier for me to grow trout since they are native to here so temperature wouldn't be an issue, but I don't really like the taste of trout. My family goes though a lot of talapia, and we all really enjoy it, so I would really love to find a way to grow talapia.
My plan was to intall dampers so I could control whether the cob mass was heated, the pond, or both. The walls of the tank will be built with cinder blocks, and bricks, with probably rocks cemented around the outside for additional support. I planned on making a cover for it, but have not yet figured out the details. I was thinking of using plywood with a layer of some sort of styroform. I think the place where I will lose the most heat will be through the grow beds, and to be honest, I have no idea what is the best way to deal with that aside from maintaining a high air temperature.
I will post with the progress of my project, thanks everyone!