Gypsum being effective "50% of the time" "I think" is a super scientific way to describe it. If you're testing hardness and completing turbidity testing before applying it, you know exactly how much you need and whether or not it will be effective.
Also calculate the amount of hydrated lime you will need during application into your cost when using alum (unless you want to pH kill of the BOW), as well as the PPE that I would highly recommend unless you want to be inhaling them both the whole time.