I'm from the uk. I can't fit anymore tanks indoors so I've toyed with the idea of an outside "room" for tanks, or a pond in the garden. If you go the cold water route you're laughing, no heating costs for the tanks or the room. If you go the tropical route then the up and down nature of our temps over here will be an ongoing costly hassle, especially with the household fuel bills as they are currently.
Your proposed rudimentary building will, as mentioned, be like a greenhouse in summer, as hot as hell, especially if you have a dark tarp.
Winter, and you'd have to have the heating on constantly in your room, or the heaters in your tanks on virtually all the time.
Personally I think you'd get pretty sick of it and pretty quick. But like I said, if you go cold water you're laughing.