My pH is too High

[biloyp]

I am growing lettuce and spinach in a DWC. I transplanted my seedlings to the DWC 3 days ago. They (6 plants) all seem to be doing good, about 1-2" tall. I took TDC and pH readings when I started and the pH was 7 and the TDC was about 1200 ppm. Today I just checked and my TDC is about 1170 and pH is 8.6. I know I can add white vinegar to lower my pH but I think I should wait until tomorrow and take another reading.

Any suggestions?