Dear Power Towel,
although your stories of tap water are quite intriguing and interesting, there are so many variables that you are not accounting for which could explain your pH change since the solutes in your water are completely unknown, the quantities are unknown, in addition to various gases evaporating over the time frame you are talking about. It is for this reason that Reverse Osmosis machines are used in growing: unless you know 100% EXACTLY what is in your nutrient solution, there can always be nutrient lockout and pH fluctuations. For example, If there is too much calcium present in your water, it may stop the absorption of any number of key elements needed for plant growth, leaving you with a deficient looking plant even though you HAVE put more than enough nutrients in your res to keep the plant healthy. In addition, chlorine and chloramines (both of which are present in high levels in almost every sample of tap water no matter what location) are toxic to a plants root system, and often promote browning on the tips of leaves, not to mention slowed down nutrient uptake! If a reverse osmosis filter is out of your price range ($100 - $200) then a detoxifying agent from any aquarium store will do. These solutions detoxify the chlorine and chloramines (harder of the two to take out of water) in addition to neutralizing the metals already in there. Put this agent in your resoivoir before adding nutes, let it bubble for a few hours, add your nutes, pH, enjoy
Also, Dear SIV3L, YOU ARE CORRECT
that zyme products are really great for your system, but I don't think its for the purpose of being able to handle pH changes. For the most part, enzyme products like....
sensizym:
http://www.hydrogardencenter.com/sensizym1l.aspx
hygrozyme:
http://www.hydrogardencenter.com/hygrozyme500ml.aspx
will help break down dead proteins, and act mainly to keep your system clean and uptake as much food as possible! Hope this helps your understanding of these products and if you need any questions answered don't hesitate to ask!