I think the insulated foundation wall won't help at all at 6". Your chance at success will depend on how much heat [from the heater] you can keep from escaping the GH. Some GH makers list how many btu's it takes to achieve temps depending on your zone. If you can find that it will give you an estimate on the size of heater you need. The cubic feet [not square feet] of the GH determines this. Will need to size your heater for the fish tank also to adjust the delta between GH temp and water temp.
