PH drop

Quick question...my ph dropped from5.9 to 4.5 the last 2 nights... wondering what might cause this fluxuation ...ppm rose from 12 to 1500. 3 afghan kush ryders and 3 diesel ryders 6 weeks old.Lookin good and very stinky!!! Usually I have to add ph down!!! Thanks!!! PEACE

As your plant takes in water it leaves nutrients behind in the rez increasing PPM and lowering PH