When ph drops in the res that quickly it’s usually caused by a small number of things in my experience. You have bacteria or algea in your res that’s releasing exudates that are altering ph, your plants are drinking water too fast, or you are evaporating water too quickly with your aeration.
Are/is your ppms/ec going up, down, or staying the same?
Another possibility is that elements in your tap water are combining with your nutrients and falling out of solution.
Yes, adding too much ph up/down can skew your ratios and cause issues. 2ml per day doesn’t sound too bad though honestly. 5.8 is ideal but bouncing between 6.5 and 5.5 is perfectly acceptable.
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