you have to keep playing with the mix and testing
too much work buy an 8.3 maine buffer

I use the Kent marine buffer at night. Had to increase the dosage pretty much to compensate for the sump.

Sodium Bicarbonate has better results through for ph bufferiing. HOw do you feel about UV Sterilizers? I have one that I haven't used in years.

As far as the buffer i use seachem and have good results with it
as far as the uv, I use to use them on my tanks and I see a slight difference but not dramatic.
I do still use it on my pond, with the sunlight u need it

Instead of posting

https://www.youtube.com/watch?v=gTmIbl20GwM


post

and this is what it would look like

I see a gauge on your filter, what is your incoming PSI?

buffer
 

i use sechem 8.3 with good success