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If it didn't stink when you got it, and it does now, something has gone bad and it should not be used. I have no idea what would have happened to it, but I wouldn't use it.
I work at a corporate office of one of the largest pet supply companies. I recently had this come up with a similiar product. The manufacturers are now using sulphur in their products because of its ability to break down nitrites. I have been informed that it won't hurt your tank, as fowl as it smells, but I don't trust it. Thats just me, I don't like adding unneccessary chemicals to the water. Good luck