Sometimes a drastic change in water quality, even if it is going from bad to better, can be enough to kill a sensitive animal like a tadpole. Sometimes dechlorinating the water is not enough. My girlfriend raised tadpoles and had high mortality using dechlorinated tapwater from where she lived. She switched to tap water from another city by carrying one gallon jugs, and the mortality leveled off. And what is good for your other frogs may not necessarily the good for these tadpoles. Other than that, who knows?