Right now it's a question of economy and transition, really. Lots changing at once, and I'm trying to keep up.
For a couple of years I've kept soil bottomed tanks that grew without too much issue, no CO2, and no ferts beyond what the soil and water supplied.
My oto breeding efforts eventually led to wanting a low pH tank, and the only way that's going to happen easily without CO2 is with a non-soil based tank (since any of the non fertilized soils I tested kept KH way too high to have an acid tank, and my tap water is liquid rock).
I chose a peat substrate, and it's worked very well thus far. Being no more than a tannin and carbon source, I needed to adopt some form of fert regime.
My readings on EI and PPS Pro led me to believe the former was more suited to a CO2 supplemented regime.
Combined with that, the output of my RO setup is about 6.5 gal/hr, and I just picked up a 75 gal tank. 50% water changes, especially with a 20-55 gal sump, would need a WAY better RO machine.
Thus, where I am right now. Still learning, but new on the curve, so to speak.
Why not split the difference between the old dose and the new dose?
Say, five or six ml instead of seven, or eight...if you rounded up.
Seems sensible, at least until the plants take off. My swords, in particular, are really liking the change from Walstad to PPS. In fairness, the otos' food isn't what Ms Walstad talked about using with her method.