Fablau sayd it all.
I was strugling with this issue for months, injecting and injecting, making changes here and there bla bla and guess what.. i follow Clive advise (Ceg4048) I bought a pump, put it right next to the outflow, both poiting into diffuser and in a couple of hours, for the first time i reach a drop of 1 PPm (7.6 to 6.6) ... easy as simple.
Taking a good or closer look i can see the astonishing diferences in the aqua.... mostly right next to the substrate.full of bubbles and the plants with bubbles attached all over them
That was quite a diference! Oh if it was!!!
Conclusion: Lack of waterflow never let me have a one ppm drop in the PH cause i was not dissolving it, but wasting it!!. And even, my diffuser is a prety dump... by the time my ADA Diffuser arrived i should be capable of decreasing a little bit more what is coming from the fire exting. Cause with a diffuser like this ADA the bubbles will be broken even better
Now, not only i have to decrease closing the valve but i finally got that drop!
I can see the bubbles going all the way closest to the substrate and not up there!. Not going up as before. Just a little insignificant parcel.
And of course plants dancing a little bit more cause ot increasing flow but... who cares? As long they get what they need, and grow beautifully, with colours let them dance !!
For me it was this simple to solve this "drop" issue.
Best regards,