Dear Ron - Manganese Greensand is a type of filter media that is made from a mined greensand mineral that is coated with a thin coating of manganese dioxide. Greensand-Plus, made by Inversand Corp., is a type of manganese greensand that has a silica core and is a little more flexible to use and works under wider operating conditions than the traditional greensand.
However, both of these media work better if the pH is in the range of 6.8 to 7.8. The minimum pH level recommended is 6.2. At a pH level of 6.1, the iron in the water is dissolved and the water may appear clear. Oxidation (a process where the clear water iron turns to a solid rust particle) of the iron is necessary for it to be removed by the filter. Oxidation occurs more easily in the pH range of 6.8 to 7.8. A higher pH of 8.0 or greater causes other problems and may cause the iron to turn into a colloidal iron which is difficult to remove. Therefore for greensand iron filters we usually try to have a pH of 6.8 to 7.8 for best performance.
Another problem with low pH is that the greensand media does not last as long. The manganese oxide coating can easily wear off and the media can become ruined when the pH of the water is acidic.
I recommend you use a calcite neutralizer or a soda ash feeder ahead of the greensand filter to neutralize the pH to 6.8 - 7.5. A calcite neutralizer is perhaps the easiest way to raise the pH to neutral in this application.