Some teas have tannic acid in them but they also have a lot of alkaloids, phenolics, catechins etc. etc. There are literally hundreds of natural chemicals in teas and coffees. They may taste good to us but any one of them can easily kill a little fish. Try it with a sacrificial guppy first, if you must. The caffeine may overstimulate their little hearts to a stop. Even de-caf products have some caffeine in them. There are safer methods of getting tannic acid into your tanks...like peat! :-) Also, peat has other beneficial components than just tannic acid.