Hello Avon211,
I been in flower transition for 2wks now. During this phase
the pH, is much more active than in the 24 hours light-time. I'm finding
the big reservoir, goes " sour " within 24-48 hours ! Now, this has alot to
do with the respiration, the plants are doing at this time. The things that
contribute to this are, the HUMIDITY in the " G-room "
TEMPS. = lights and ventilation

Plus, the type ( strain dominance ) of your plants ! In my G-Room they are,
Sativa dominant, so my setup is to have the lowest HUMIDITY possible.
Keeping them on this " edge " is risky and uses more
of everything ( water & nutes ), cuz'n, THAT, is what makes the big reservoir
go " sour " ! When the plants aren't perspiring, they are dumping hydrogen ions
into the water, causing the raise in pH uh!

toke,
practice,
Zeffern