Promising review: "Finished using it five minutes ago and it’s the best! My husband uses a Stanley thermos daily to take coffee to work. He tries to remember to rinse it out every day, but that doesn’t always happen. I got the package today, filled the thermos with water, dropped in a tablet, and let it sit. I promptly forgot about it for a few hours. Initially, I was disappointed when I dumped the water, as it was barely tinged brown. Then I put fresh water in, covered the top, shook it, and OMG. The crap that came out was disgusting, yet fascinating. I looked in the thermos and saw almost nothing but sparkly silver! There was a little crud left in the top third of the thermos, but it’s a really tall one, so I wasn’t surprised. Got the bottle brush, swished twice, and bam! All clean! No fumes, no smell, no nothing, just clean. I used to struggle cleaning it with baking soda, vinegar, the brush, soap, and elbow grease. Screw that. I will forever use only these tablets! Here’s to better tasting (and way less gross) coffee!" —BranchingOut
Get it from Amazon for $8+ (available in packs of 12 or 36).