That was one of my first hunches but if that was the case would it not be affecting the thousands of small fry and other critters that inhabit the shallows. I was wondering if some sort of gas could have built up near the bottom do to stratification then as the pond became much shallower due to the drought it got brought up quickly because of the last very windy days and briefly mixed throughout the water. I know frogs are very sensitive about absorbing toxins threw their skin and am just assuming tadpoles would be the same. The frogs could have left the pond to escape it but the tadpoles could not. We did notice a slight odor and see a very thin "film" on the water tonight but I guess that could have also been from all the dead tadpoles. I dont have a clue really just throwing out hunches. My biggest fear is that it may be just the beginning as in the most sensitive species dieng off first due to DO or heat related problems.