I use hydroton as medium. This is my 3rd time that I am using the same hydroton. 1st grow the ph rise was 2.00 - 3.00 per day. 2nd grow was always 1.00 per day. Now 3rd grow is 0,4 0,6 per day. I agree with jondamon you have to wash your hydroton very well and if possible let them rest 1 or 2...