I've acquired a handful of different size carboys over the years and all of them are serviceable, but one has some deposits/staining inside. It's near the top of the vessel, a few inches down from the neck and looks sort of like hard water deposits. I'm not sure what it is, but I know that the following haven't removed it: hot soap/water + carboy brush, bleach water, white vinegar, muriatic (dilute HCl) acid. I've brewed in this vessel, and I don't believe the deposits are going to have an adverse affect on the product, but the obsessive side of me would like to get it cleaned up. Any tips on stubborn carboy stains/deposits?
Hot PBW (Powdered Brewery Wash from Five Star Chemcials) takes off everything I've thrown at it.
For carboys, bottles, and bottling buckets I've had good success with OxyClean. I always rinse and sanitize right after.
A 20-minute soak in Diversol has never let me down. It sanitizes, too!