I have not used that product myself, but a quick search on thorite, the main ingredient of said product revealed this
Thorite - Wikipedia, the free encyclopedia
This article and several others I brought up claim that thorite is radioactive. I am not a chemist or anything, but would research it further even if it is considered safe for fish tanks ( can hardly believe it is considered safe for fish tanks after the things I have read about it). Hopefully someone else can elaborate further or prove me wrong.