Ph rising in dwc overnight.

[Dougler]

Hello everyone. I'm new to hydro and having some issues with my dwc system and just looking for some insight in to what might be going on.

My setup includes a 5 gallon bucket with netpot(washed with hot water before installing), hydroton(rinsed until run off was clear and soaked in 1/5 strength nutrient RO water for 24 hours), airstone(Soaked for 24 hours in plain RO water) and air pump. Before I put any plants in there I wanted to run the system for a few weeks to get it stable. When I run the system overnight the ph seems to want to jump up close to 7. Below are the particulars.

Starting with 5 gallons of RO water of ph 7 and ppm of 5.
Added 2 teaspoons of pure epsom salts, 150ppm.
Added GH 3 part nutrient, 10ml of each (less than half recommended strength). Mixing thoroughly between each part. Ph of 5.5 and ppm of 500.
Added 3.5ml of Enzyme Komplete as recommended by my local hydro shop.

I let the water sit overnight before taking a final reading and got a ph of 5.6 and ppm of 550.

I added it to my system and ran it overnight. Air on full and hydroton in. Reading first thing in the morning was ph 6.5 and ppm of 560. Checked it after 24 hours and ph is at 7 and ppm still at 560.

I'm not quite sure what could be causing this. I've noticed the hydroton runoff was higher than the starting ph but I've read and been told that the hydroton will not rise or lower ph.

Any help or any more information needed would be greatly appreciated. Thank you.