I'd like to find out why your running so hot. Maybe skip the aquarium water for a round and test the runoff? Aren't most aquariums at 6.2 with constant filtering? I'm not sure how this would effect anything, but I don't have experience with aquarium water, or alfalfa meal - You are of course testing the PH after you add your nutrients, right?

I know that sounds like a silly question, but giong from a ph of 6 to a ph of 8, either means something is seriously wrong with your soil, or you have the magic radiation water. And if you've got an aquarium, I'm doubting your water is radioactive, makes me think of the soil. If your in the middle of BFE and got your soil from a home improvement store, was it in the outdoor section of the home improvement store?

Why do I ask? Because if there is a chance the soil got wet while it was in the bag, it starts to produce all sorts of little nasties, from the mold, which also causes nutes to ferment. This could cause a rapid change in PH.

Test the soil.