controlling PH

I am new to hydroponic systems and having trouble keeping PH in the recommended range (5.2- 6.0?) . every time i add a ph down solution, it will go down temporarily but within the hour it will go right back up to 7... any suggestions

I have a couple questions, which may help things along:

What source of water are you using? City tap water, reverse osmosis filtered or distilled water, bottled or drinking water?

Also, what nutrients are you using? Brand/formula?

ph test your runoff water, and when you are adding your phed water, ph down it low enough so that your runoff is at the desired ph. a few weeks ago when i was using 1/4 strength nutes, i had to ph it down to 4.3 to get my runoff to the proper level (5.4-5.9). now, even at 3/4 strength nutes i barely have to add any ph down...

also what are you using to get ph down?