Huge pH drift IN HYDRO please don't tell me its the fertilizer

The dissolution of CO2 in water changes its chemistry. Distilled water readily absorbs carbon dioxide (CO2) from the air, forming carbonic acid and making the water slightly acidic due to this absorption process; Because distilled water is essentially free of minerals, it quickly absorbs CO2 from the surrounding air. If your environment is rich in co2 it could be exasperating the drift. Only thing I can think 🤔. If its not dodgy fert. Good luck.

Maybe something else is growing in the res/tubes? have you heard of products that keep stuff clean? addatives you add into the res to help fight off microboes and fungi that mess stuff up. Mostly Hypochlorous acid or peroxide. most brands have a product.

You’re going to get drifts like that in hydro no matter what nutrients you use. I change my reservoir once a week with each new feeding, or more depending on the size of the plant. I use Terra Aquatica nutrients. As the plant uses the nutrients a byproduct is produced (salts) that build up in your water causing this fluctuation. You have to stay on top of it, keep it between 5.8 and 6.2 for the best results. If you check your ppm on the regular you’ll find how the plant eats these nuts daily, and can be more aware of when to change your reservoir. If your not changing your reservoir regularly your headed for problems. The PH is your number 1 priority in DWC, if you don’t keep it right, everything will go wrong. There are some nutrients better than others out there, but they all need buffering.

I can't speak for Hesi but in my experience in DWC using GH Flora Trio, once my rez water is oxygenated and adjusted with down the only real changes in pH I see is through either evaporation or the plants drinking more water than nutes(aka my EC rises) so it's simple to correct when topping off between tank changes. I generally would be adding either just straight water or half strength nutes depending on my readings.

Good hydro fertilizers are buffered to resist pH drift. As long as you can rule out temperatures, light pollution, or something odd things added, better fertlzer might help.

Damn, that pH drift is brutal. You're spot on about evaporation. But dropping to 4.8 after a reservoir change is wild. Double-check your pH meter calibration first. That's the most common culprit. Also, what water are you using? If it's tap water, try using RO or distilled. It sounds like something's throwing off the buffer capacity. Hesi nutes themselves shouldn't cause this.