Tannins are polyphenols that are released by composing leaf litter found in most bodies of fresh water, especially those found in the tropics. If you have ever seen water that is tea in colour, then you have seen water that contains these polyphenols. This black water, as it is sometimes called, is usually on the acidic side as far as the pH is concerned and is ideal for raising tadpoles in, especially those of rainforest origin.
To introduce tannins to your water you can do it by one of two ways, you either use "black water extract" or, as I have done, place almond leaves in the water. Doing the latter also provides some natural nutrition for your tadpoles as I discovered. My red-eye tadpoles have been feeding on this large almond leaf 24 hours a day since I put it in. Now there is hardly anything left of it.
Black Jungle sells these dried leaves, their site - http://www.blackjungle.com, along with "black water extract".
*Looking over the site, I did not find the almond leaves, but they do sell them, I know because I have bought them from them.
Try feeding the tadpoles with flaked fish food, Omega One is a good brand to use.
What kind of filtration are you using? It should be strong enough to clean the water, without bashing the tadpoles around.