It's not the hydroton; it's your prep method.
The stuff is the cat's ass; you just have to 'set' the pH before adding plants. Run your system with no plants for a few days, whcih you have to do anyway to check for leaks and determine your flood/drain timing. During that time, check the pH in the res 2x/day and bring it down to 5.8-6.0 with Nitric or Phosphoric acid. When it stops dropping, do a complete res change, add your plants and nutes, and rock on.