what type of acid are you using to lower your PH?

how are you testing the PH?

is the temperature of the water changing?

I'm using phosphoric acid, im testing the pH with a digital tester, a hanna. The tempreture of the water generally stays cool but never fluctuates alot.

Hydroponic nutrient's Ph increases as nitrogen is used.

Keep a daily log and you will see how Ph and Ec are related to plant type and growth stage.

With nutrient temps below 75F, add Ph down until Ph is correct.

It sounds as if you have carbonates as suspended solids in your water. Carbonates act as buffers to raise the PH. A common occurance where limestone exists.
If you use enough phosphoric acid, it will stay down, but will go back up when more water is added or you could add ammonium nitrate as about 15 to 20% of your nitrogen source. This will lower the PH as the nitrogen is being used by the plants. Using more than 20% ammonium nitrate could damage your plant roots.
Also, unless you are doing some type of experiment, don't worry about the PH being too high as long as it stays below 7 and your plants do not show an iron shortage.