You have a filter version Fleck 5600 12 day time clock control valve. I don't see a top dome hole plug in the tank so you would remove the filter from the bypass valve and unscrew the control valve from the tank and add mineral and put things back as they were.
To know what mineral you would need a water test for pH on the raw (prefiter) water. You can buy the mineral at most local independent dealers or plumbing supply house. You find either in your Yellow pages under the headings Water and Plumbing.