Large ph drop

[morrum]

We would be interested in knowing possible causes why in a hydroponics system our ph is almost like clock work dropping from a ph of about 6.8 to about a 6.4 to 6.3 in about 12hrs and has been doing that for several day while EC amd PPM are fine...the plant still looks fine with no possible signs of any problems.

Thanks for any imput you may have