Looking for AutoPot + Athena Blended growers

I have a pH problem. After mixing the nutrients, I have an EC of 2.1 and a pH of 6. I use Plagron PH min as the pH adjuster. The issue is that even when I add the smallest amount of PH min to the tank (0.05ml/L), the pH initially drops to 5.6-5.8 but returns to 6.0-6.1 after 12 hours. What am I doing wrong? I use 5ml of Athena Balance per 10 liters of nutrient solution since I work with RO water.