Thats good shit, I dont know if you can use it in the water itself but I use it with the soil. It works good. Not sure how much, I just keep adding it till I can see its in there when im done.
This is what my book says: MARIJUANA OUTDOOR GROWERS GUIDE, by S.T. ONER...
"Wood ashes work very quickly to raise the ph, are high in micronutreints and are, in general, just good for the soil."