Are you on city water or well? Please refer to the chart I posted.I think @JD77 found the answer to my issue.
Yesterday, I did another reservoir change. I waited four hours or so before taking the following readings:
Air temp: 67
Water temp: 65.3
Humidity: 32%
PH: 5.8
PPM: 550
Today, about 16 hours later, I took the following readings:
PH: 4.8
PPM: 560
Water level in my 5-gallon reservoir dropped by about a pint.
When did this daily drop start happening? About a month or so ago, right about the time I turned on the gas furnace for the winter season. And where does my furnace sit? About 15 feet away from my grow cabinet.
So now I have to decide: Do I continue to use RO water, knowing that it sucks up CO2 like a sponge? I would have to continue to add 1.0 to 1.5 ML of PH-Up to the reservoir daily, not knowing how this might impact the plant. Or do I change to tap water, which would hopefully be more PH stable, not knowing what exactly is in the water to start with and having to deal with chlorine / chloramine?
You are experiencing leaching and since it's relatively minimal I would lower the ppm by 100. Monitor the situation if it stabilizes great if not continue to drop ppm by small increments until the problem resloves itself.
Res changes aren't needed to adjust. simply add some water to dilute the concentration of nutes in your res!
Viola!
Hope this get a you going in the correct direction!