Yes, it has to be above ground, cause I have a house around it and can't have underground rooms. But as all the tank walls will be interior (the tank will be surrounded by a house), termic isolation will be easier.
It seems that controlling temperature is one of the most important issues here.
The maximum temperature variation during a single day can be around 20 degrees. But this amount of water won't follow the outside temperature fast, winter or summer.
So what I need to worry about are the average temperatures. During summer, if it gets too high, the tank's roof can be covered with white, letting some diffuse light in.
I'm also trying to figure out an easy and not expensive way to control the sun light it gets as well as isollating the tank's roof termically.
As for heating during the winter, I think that it's possible to have a main system based on solar collectors (the government pays part of their initial cost here) and another extra form of heating, like the pool heating you suggested. This has to be combined with the good termic isolation, specially in the tanks roof.
I was thinking if my initial idea of making the water run all the way through the house's roof couldn't be adapted to help heating in colder days. A way to automate this would be to have the water temperature checked when entering and when exiting the roof. If the water was colder after leaving, the pumps would stop sending water through the roof.
My initial idea was to have the water going through a 50 meter course, 3-5 inches high, around 5 meters wide. This could be used both as filter and heat control. For instance, covering the roof area with a white material during summer and black during winter.
