first you need your rez. Then you need a control rez. This is usually the same type container. These are hooked together with a pipe hard or soft doesn't matter. These two will seek the same level. A small pump in the grower rez to send water to the controll rez and now it recirculates. You can keep it topped up and check the PH at the control rez.

Now if you add another fill rez. This would be the third rez which sits on top of the controll rez to feed it fresh water. A float valve like a toilet is put in the control rez at the hieght you want your water level. If you had a PH dowser you could walk away for days with out worry's.

Take a look at this one and you will see all you need.
SH Hydroponics, Inc. > Reservoirs