Because you are using tap water, you have to live with a small problem of maintaining PH. This is especially true at the beginning when you're not using very many nutes. As your plants get bigger and you add more nutes to the water, you'll find that the PH problem is much more managable.

I find that keeping normal tap water PH below six in a 55 gal resevoire takes me 15 ML of PH down per day when the nutes are light.

The problem will go away as you increase the nute level. If you really want to get the problem under control, get a Reverse Osmosis pump/filter.

http://www.excelwater.com/eng/b2c/water_tech_4.php

GM