I have a set of 16 American solar works evacuated tube collectors (400 total tubes) which provide heat to a 900 gallon fiberglass tank.  Inside this tank, I have a copper exchanger at the bottom coming from the solar collectors and three copper exchangers above it: one for DHW, one for my radiant heating floor, and one unused.  The tank has had R11 fiberglass insulation wrapped around it, and it was installed sitting on a concrete floor in a mechanical room which is within the conditioned space of the home.  How would you suggest increasing the insulation on the tank?  I drain it regularly to replace the water, and the tank could be rotated to facilitate insulating the surfaces, but it would be difficult to move it outside.  The tank is regularly 130F-150F in the evening during the year.

How should I insulate below the tank?

How should I insulate the walls of the tank?  I have thought about 2 or more inches of 2lb spray foam.  I have also considered constructing a small wall around the tank and filling it with insulation, but it is less than 12 inches from walls or equipment on 3 sides.  Are there any other types of insulation to consider?  Would a radiant barrier paint or ceramic paint be beneficial prior to adding other insulation?  I am interested in the best possible system.

How often should the water be changed?
Are there any chemicals which should be added to the water?

John