As far as I am aware when I was researching the same thing after discovering the nitrate issues in my water it is a product similar to activated carbon in a filter. It does not actually release any form of chemicals into the water. And thus theoretically should be safe for amphibians.
When I was researching it I saw mixed reviews about its usefulness. Apparently it can be re-used several times so its worth its money. Also one review I read, if I remember right, said that the website states you must remove it once the nitrates have been eliminated, and not leave it in the tank. So basically whenever you need nitrates removing, pop it in for 24 hours then remove it and recharge for next time. theoretically you could use one bag for all of your tanks at home in succession. Recharging is supposed to be easy too, apparently just salted water.
I say give it a go. I may do so at some point with my fish aquarium.