To the best of my knowledge cement will leech toxins into the water and make it extremely dangerous to amphibians among other things. People who use Cement are usually advised to seal the cement in Drylock type paint to seal it as good as possible.

The cement is the only environmental thing you listed that I see that could have caused this.

I am so sorry for your loss. If you haven't already I would pull the last frog out of the tank.