Cant ph my water please help

[MrGreenfinga]

So i got a 15 liter DWC resivoir
And everytime i put PH Down phosporic/Nitrate Acid into it
The day after it bounces from 5,5-6,5
Im using the flora Tripart nutrients with the GH ph up and down

have no idea what im doing wrong and how it changes this fast please help

 

[Zill]

Mr. Green,

15 liters. Have you calculated the amount of pH down you would need? Seems to me you need to incorporate a manual add/ mix cycle. When you add the pH down stuff is the water circulating? Do you live in area with high water hardness? What is the pH of the incoming water?