pH Dropping 5pts in 24hrs...

This was the problem I hope, For the last week I been fighting a problem in my NFT Gutters the rez was dropping 5pts over night no mater what I did I would just continue to drop.Its been 3wks running, I cleaned the rez lowered the nutes to 1/4 strength , cooled the water to 70-72 degrees, made sure no light got in the rez and changed the water every 3days. Nothing seemed to help, then I was just checking out the root growth and low and behold there it was, all the black mold was on the plastic that is wrapped around the rockwool cube. I should have known better the water was flowing 24/7 and caused this mold to build up. Its not been a full 24hrs since I change the water after I found the mold and the nutes are staying 100% more stable then they have been for the past wk. Just thought I would share the info in case someone is having a problem with their pH going down.