Not being around froggies much, I had to refresh my memory:
http://www.ypte.org.uk/environmental...nd-tadpoles/28
Another test could be to use different water and see if a different environment makes a difference. (Note: If starting with tap water, it must stand for 3 days min before putting the little squirmies in)
Where are you getting the tadpoles - aren't they more of a spring crop?
This is fun - a group project.