If you've recently cleaned your tank, it could be residual soap from the cleaning process. I had issues cleaning a Subtank where I just could not get the taste of honeydew melon out of the tank. No matter how often I cleaned it and what new flavor I filled it with, the taste of melon would not go away. I finally had to remove all the o-rings and soak them in alcohol overnight. That did the trick. So, you may have something in your o-rings that is affecting the taste.