The problem is in the res. you need to keep that constant becouse as the water and nutriets get used the ph rises its prettly simple you can use this controler but i recommind a more percision dossing pump than that one you have to dilute or you could use to much to fast. If you have the cash use one of these and if you really do buy the all in one its as simple as plugging it in, and if you have know how build it super easy. This is just one good place to find these pumps and they make their own systems.
http://www.automatedaquariums.com/h_sys.htm

they also sell just the pump which can be used with the milwake for i think 75