here you go megalith:
http://forums.gardenweb.com/forums/load/hydro/msg100749279742.html
I had a similar problem and there are a shit ton of links about hydroton jacking ph if not properly treated.
Here is the meat of the thread (as a flame war erupted):
Hi Lucas, I think you are underestimating the capacity of cation exchange to change pH. Clays - especially those that have been dry for long periods of time - tend to be very basic and they will buffer a solution (especially if its poorly buffered) from 6.8 to even 9 in overnight. Also clays do increase their basic activity if they are stored for prolonged periods of time due the oxidative activity of atmospheric oxygen that creates basic defects within the crystal structure. Of course, this depends on the actual nature and composition of the clay but it does happen with certain frequency.
In commercial operations where media needs to be buffered the easiest thing to do is to first run a pH 3-4.5 buffer over the media within the hydroponic setup for a few days, adding more buffering agent if the pH drifts. When the pH remains constant the solution is ready. Usually good these buffering agents include acetate/actic acid and phosphoric acid/hydrogen phosphate but others such as citric acid species may also be used. Especially since they also have the ability to chelate cations and make the process faster. The initial buffer is usually prepared at 0.5 mM but the concentration can be increased if needed. After this the system is flushed and the crop is started.