Hi,
The extra frame is not a bad idea -- there is a lot of water pressure on the tank sides. Be careful to make sure that on the perimeter frame corners that the east-west frame member has a good strong shear connection to the north-south frame member -- a full glued lap joint + screws, or flat metal splice plates.
Since you are keeping the temps fairly low, you might be able to just insulate on the outside of the plywood tank walls. The plywood wall has about an R 0.75, so the temperature will drop across it some -- this might be enough to get it to where the extruded polystyrene insulation would be OK. But, the bad things about this are that 1) insulation on the inside of the plywood will be more effective and easier since there will not be any thermal bridges from the perimeter frames, and 2) if the temp ever does get high in the tank, then you could damage the polystyrene.
Better to find the polyiso if you can. If you have some independent lumber yards in the area, you might try them. Both of the two places that carry polyiso in Bozeman are independents -- our HD and Lowes don't carry it.
If you can find the 2 inch polyiso, I would put 2 inches inside as long at that leaves room for the pipe coil heat exchanger you are using -- this gives you close to R14 with no thermal bridging.
Please let us know how it comes out -- if you post it in the diaries section, maybe you can drop me an email when you do.
Gary