I have a small tree purchased from a mail order company that was labeled tangerine. Once it produced fruit this past season, I got to taste them for the first time, and they were horribly bitter! Is there a secret to sweetening container grown tangerine? They are blooming again now in the greenhouse.

Sounds as though you may have bought a mis-labeled Calamondin - an unpleasant surprise if you are expecting a tangerine. Providing pictures of the fruit and tree might help us identify it.
If it is a calamondin, they do make attractive houseplants and calamondin-ade is excellent.

Your question is frequently asked on this forum. What you experienced with the early fruit taste is very, very common. You cannot expect a young tree to produce good fruit in its first year. It is normal for tangerines (mandarins), and many of the citrus varieties. It takes approximately 5 years before a citrus tree begins to produce its best fruit, but each year the taste should improve. - Millet

Thanks all. We will hope for better results in the future!