I don't know why your tadpoles are slow to develop but I can tell you that a friend of mine has a similar situation. He built his small concrete pond in the shade of a large laburnum tree, the leaves and seeds of which are toxic. He doesn't keep fish and the plants don't seem to mind the poison, but the tadpoles never seem to grow up. I'm half-tempted to try a glass or two of his pond water to see if it works on the human aging process. (Not really, don't try this at home kids!)