How fast does the change occur?
Is there anything else in the tank and what is the tank made of?
Are you adding anything to it including nutes?
Does the water drain back into the tank after watering your ladies?

Here is what I found when I googled "hydroton ph rising problems":
Rising PH Problems? - GreenPassion.org - Dedicated to Medicinal Cannabis Cultivation and Education

I hear it all the time...........the grower, sets up his tank, carefully adds the appropiate PPM of nutrients to the solution, PH's the system, and adjusts the PH as necessary.

Than the shit hits the fan, within a few hours of setting up the tank, the PH rises dramatically, maybe up to a PH of 8. The grower has no idea why either, they go crazy adjusting the PH again, and still it rises again. You gotta wonder, what gives, right?

I'll tell you what gives.....it's the rock wool!
You HAVE to prepare the rock wool, prior to using it.

Rock wool must be bathed in an acid solution before being used to bring itâ??s normally high pH down. This is done by soaking it in a nitric or phosphoric acid solution with a pH of 4.0 for a day or so. You must still continue measuring and adjusting the pH of your solution to keep the pH at the root system around 5.5 after you put it to use, but it should be much easier. The pH of your solution is rising because as it circulates through the system, the high pH of the rock wool is bringing it up little by little. You also need to make sure that your nutrient solution is properly aerated and the ppm is stable as the interaction of all these factors can affect the pH in your system.

The easiest answer is to use PH'd water:
Everybody should soak the rockwool in no lower than pH 5 water the day before use. This is done to dissolve the lime. The lime will make the pH value raise a full point.

Immediately before use; you flush the rockwool with your nutrient solution. When you flush, you also flush out the dissolved lime.

From this point onwards rockwool does not change the pH in any way. Exception: If you condition your rockwool with a pH lower that 5, you may also damage the fiber itself. As an example: If you use pH 4 water for conditioning, the pH the day after will be 7. The lower pH you use the higher it jumps. Once the fibers are damaged it might take quite a while before the pH stabilizes. So never go below pH 5 with rockwool.