Quick take
Test iron, manganese, copper and pH before choosing a filter. Low-pH water can create a plumbing-corrosion problem that filtration alone does not solve.
Iron and manganese
Dissolved iron can create metallic taste before visible staining appears. Manganese can contribute taste and dark deposits.
Copper and corrosive water
Low-pH or otherwise corrosive water can dissolve copper and other plumbing metals. In that case, neutralization may be as important as downstream filtration.
Why a TDS meter is not enough
TDS cannot identify which metal is present. Laboratory testing is needed when a specific metal drives the treatment decision.
Treatment paths
Iron/manganese filtration, neutralization and point-of-use RO can all be appropriate depending on the measured chemistry.