If stains are blue-green, test copper and review pH/alkalinity before buying filtration. A filter may reduce copper at one tap, but it does not correct corrosive water attacking plumbing throughout the house.
Why copper creates blue-green stains
Copper plumbing can release copper when water is corrosive. Deposits can appear blue-green on sinks, tubs, drains and fixtures. A metallic taste may appear too, but taste alone cannot measure copper concentration.
Use the pattern to narrow the source
If staining appears throughout the home, water chemistry and broad copper plumbing exposure become more plausible. If it is isolated to one fixture, local plumbing or a specific fitting deserves attention first.
What to test
A useful test can include copper, pH and other corrosion-relevant chemistry. Sampling instructions matter because first-draw and flushed samples can answer different questions about premise plumbing.
Treatment paths
For drinking water, an exact model with a verified copper-reduction claim can reduce exposure at the treated tap. Reverse osmosis can also reduce many dissolved ions. For whole-home corrosion, solve the chemistry or plumbing problem itself.