Briefly had an issue with "metallic" flavors. Its source was contact with copper post-fermentation. I was using a short piece to weigh the end of the vinyl tubing down when racking to kegs.
Tom Schmidlin cleared that one up for me.
You swap tanks? If nothing has changed except the co2 then look there first despite others' experiences. Take a batch and split it. Carb one with the current co2 and prime the other. Serve the second off-system. See if there is a difference.
After reviewing my notes , dates, etc the only common thread on the 4 kegs is the gas and I have no copper in my system or process. I am I have a second keg of two of the same beers on tap sitting chilled in a keezer so I'm going to hook them up to my second tank and sample off picnic taps in a few days and then then a couple weeks and see what happens
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