The unknown chemical affects only a small percentage of the fish in a tank. If you use it when there are only a few fishies, you may very well "waste" that dose. As I said before, it can be very useful, but it takes some (expensive) experimentation to determine how to use it well.
If you have a lot of fish dying, you'd be better off spending your money on Fish Environment research and using the method that I outlined in the post that I linked to above. It's effective and costs none of your hard-earned Isola Bucks.