Diagnosis
Mineral and organic buildup from unfiltered water or just regular use. The plastic reservoir needs a periodic deep clean the filter doesn't handle.
How to Fix It
Use a 1:1 solution of white vinegar and water. Fill the reservoir, let it soak for 15-20 minutes, scrub with a soft bottle brush (never abrasive pads), then rinse thoroughly. Run a cycle of clean water through the dispenser afterward to clear any vinegar taste.
If That Didn't Work
For stubborn film, use a paste of baking soda and water as a gentle abrasive. Scrub, rinse, then follow with the vinegar soak to disinfect and dissolve any remaining residue.
Heads Up
Never use bleach or harsh chemicals. They can degrade the plastic and leave a permanent odor that will taint your filtered water. Vinegar and baking soda are safe and effective.