I thought this should be shared. Been fighting an off flavor for the past year. My competition feedback on many brews cited the following: harsh, medical, phenolic, hop burn, etc. Not overwhelming, but just enough to keep me out of top honors. It was more noticeable in mild beers. I worked on mash and sparge temps, PH, grain crush, sanitation and grain and hop freshness. Same, same. I was at the LHBS talking about getting visible stuff in my HLT water. It looked like alge. The LHBS owner said maybe that was causing my off flavor. I started researching off flavors from alge. Sure enough, there it was, astringent or phenolic. My water is from a well with no treatment. I attach a food safe hose from an outside yard hydrant to plumbing inside my shop that runs up stairs to a utility sink. The water line feeding a water filter ahead of my HLT was black and slimy inside. The filter was a bit slimy inside also. I took everything apart, cleaned, replaced, etc. All hoses are now removed and allowed to drain and dry out. I even open the water filter, drain and rinse and soak the filter cartridge in starsan before standing to dry. All is good now. What an ordeal. Cheers and happy brewing.