If the taste changed after a filter replacement, flush the system exactly as instructed. If it changed gradually, check cartridge age, membrane rejection, storage-tank stagnation and sanitization needs.
New filters need proper flushing
Carbon fines and manufacturing preservatives can affect taste immediately after cartridge or membrane replacement. Follow the brand-specific flush procedure before judging the water.
The post-carbon filter is a common taste culprit
Many RO systems use a final carbon polishing filter after the membrane or storage tank. Once exhausted, it can stop masking tank or plumbing tastes.
Stagnation and sanitation
Water sitting in a storage tank or lines for long periods can develop stale taste. Systems with tanks also need periodic sanitation according to the manufacturer instructions.
Check membrane performance
A failing or incorrectly installed membrane can allow more dissolved material through. Trend feed and permeate TDS as a maintenance clue, but do not treat TDS alone as a contaminant safety test.