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Eleena it's the one kitchen appliance that I used to think lazy cows before I had one and now it's the most important appliance I have :-)
Never heard of tannin nor ion exchange and do't use a water softener. This has started happening when it didn't with the same products. Have had both sprayers out, they run freely and the filter. It's almost like it's doing the wash without detergent (although it is). Everything else is coming out clean to the eye but tea stains are awful.