I have a question for you guys. At the start of my grow I always had to use PH down on the make up water and Had to add a bit every couple of days to keep the PH around 6.1.
I am now 45 days in flower and I now have the problem with the PH dropping and I have to add some PH up every couple of days .
Also why does my PPM climb so much , did a nute change , into the tub went PH 6 and 1450 PPM (200PPM @ the start).
2 days later the PH is 5.7 and the PPM is 2150! Just added 5L of water and will check again later .
What kind of water and nutrients are you using? I had a similar thing going on when using future harvest. Are the plants drinking lots of water that would explain the TDS climbing so much, as well as the PH dropping because the nutrients are acidic so as the concentration gets higher the PH will drop.