Ok, if it is only moving .2, consider this a normal fluctuation. Also, you must use ppm in conjunction with ph, not one or the other. Everyday (or every other if you have a large rez) you need to top off the water to replace what your girls drank. However, all of the missing water isn't just water that they drank, but also evaporation, and other sources such as leaks..the list goes on and on. you you change your Rez and adjust it to 1400ppm and a ph of 5.8...2 days later you find you need to top the water off:
Before topping off: ppm 1550, ph 5.2
After topping off (takes 3 gallons): ppm 1400, ph 5.8
What's happening?: Your ppm is too high, the girls are drinking more than eating.
Before topping off: ppm 1200, ph 5.8
After topping off (takes 1 gallon): ppm 950, ph 6.8
What's happening?: your girls are eating more than drinking, consider upping your nutes a little.
Before topping off: ppm 1400, ph 5.8
After topping off (takes 2 gallons): ppm 1200, ph 6.0
What's happening: your girls are eating and drinking at a pretty steady rate; this is the sweet spot everyone shoots for.
I could list probably hundreds of examples, and your case isn't going to be like those above, but hopefully you see the inverse relationship that nutes/h2o have on ppm/ph. I colonize my rez with good bacteria, and they buffer the hell out of my water. I usually only have to adjust ph after I top off my Rez with fresh h2o (it's over 7 coming out of the tap, so I have to bring it down.) hope this info helps...let me know if you need further clarification.