Problems with ph dropping rapidly?

I'm growing in an aeroponic system and have been for about two years now.. I've always had awesome results but this time around the ph in my resevoir is dropping rapidly overnight.. I try to keep the water around 5.8 but it has been dropping to as low as 3.8 in a short time. I use the generic ph up and no matter how many times I correct it, it seems to want to go dangerously low.. Any ideas?

Generally a ph drop of that size, regularly, indicates a rot issue. What is ppm??

SuperStoner is spot on. What are you using for pH UP/DWN? Make sure it's not organic if you're running a sterile rez.