HYDRO 8 weeks into flower; 3 plants slow; PH drops.

[Sino]

Hello all, wanted to check in as this is still new territory for me, going hydro on these do si do. They’ve been pretty good since seed besides some learning curves here and there.
I’m 8 weeks and a few days since the flip to flower and not sure if I have issues or not. PPM has been brought down to 450 and I believe I’m aiming for around 380/400 on my next res change.
These last few flower weeks I’ve been aiming for 6.2/3 PH, I dial it in and the PH drops to 5.5/6 within a few days. I’m using PH up for the first time in my whole grow and not sure if that’s normal or not. I use an in-line bluelab that I calibrated before this post just to be sure.
Roots and plants seem ok, some nute burn here and there but nothing wild that I can notice at least, 1 out of 4 plants has been getting pretty fat while the others are taking their time, not sure if that’s normal for 8 weeks into flower or not too.
Humidity has been 40-50. Temp swings 72-78. Have 3 fans going different directions. Electric skys are at 75% power. If I missed anything please ask and thanks for the help (:

[alltatup]

Sino Plants look great! I also grow hydro, and I keep pH 5.8-6.2 . Sometimes there are big swings in the pH and sometimes not, but I check daily and most days I have to add a little pH up or down. But I consider it absolutely necessary to check daily.

[WillBlack]

I have to agree with what alltatup said above. I have been growing hydro for a bit over a year now and as the plants get bigger and they will drink up the water in the reservoirs surprisingly fast, so pH changes will happen. I also check the pH of my reservoirs twice a day and adjust accordingly if needed. If you are changing your reservoir every 7 to 10 days you will need to make sure you check the water level about mid week. You may need to be adding a gallon or two of pH'd water in between reservoir changes. Just make sure you leave about an inch of "air space" between the top of the water and the bottom of the hydro basket. Just my two cents, your plants look great! Keep it up and do your best to maintain that proper pH.

***Just realized how old this post was, sorry for the super late reply!! I hope they came out great!***