PH is dropping but just a little like it’ll be 6.0 in the morning and by night it drops to anywhere from 5.2 to 5.5. I keep adjusting by adding ph up. PPM has been staying around the same tho. Plants are drinking the water. I’m growing Autoflower northern lights 24 days old RDWC set up
Thanks 
for the help
That’s a big drop. Try to stay on top of it and shoot for stable at 5.8.
It may be something your adding that drops it.
What type of water are you using ? Tap, distilled , RO ?
When my ph was crashing it turned out I needed to clean my reservoir and tubing had slime growing, but I’ve heard that as the plants uptake certain nutes and leave others behind can cause a drop.