It's often difficult to get a natural balance in a small pond, the effect of sunlight will turn it green and may even make it go "soupy" so you won't see your tadpoles.

http://img534.imageshack.us/img534/9927/pic002a.jpg

An occasional trickle change will resolve this.

But it has to be just that, nature doesn't like rapid changes in conditions.

I do ours a couple of times in the summer. I also clear out all the dead vegetation before spring (it will seem that every leaf that falls in the garden will likely end up in your pond) as it's likely to be acidic.
Frogs can absorb toxins through their skin.