Hey erice, I am unsure as to whether this is the right 'alum'...
http://en.wikipedia.org/wiki/Alum
but if it is then here are my concerns. While it may correct your pH, acid and bass that are sold in hydroponic stores are specifically formulated to be used in hydro systems due to their inactivity with other chemicals in the water, and the safety of their use. While your system may be balanced, I cannot (and I don't any grower could without a mass spectrometer of some kind) tell you the interactions your alum will have with other chemicals in the system, and also the safety of your product is put at risk because your roots will just absorb the toxins that the alum may create, and then you are the one who harvests and eventually tastes the fruits of your labor and toxins unfortunately. In addition, while there are some home made DIY ways to lower or raise the pH without these special acids or bases, make sure that the acid or bass itself is not to strong so that when you add it in your system, it won't damage the roots. In short though, it is always better to go with a formula made for what your working with for the best results. If you have any more questions don't hesitate to ask!
http://www.hydrogardencenter.com/phdownacid8oz.aspx
