Like I said, lower CO2 and no more KH.
ADA substrate is *MEANT* to keep the pH stable between 5.8 and 6.8 pH.
Adding KH to the water column causes the pH to rise, as it helps to buffer the pH.
HOWEVER........ because you have a buffering substrate, the substrate consumes the KH in the water column. This causes the pH to lower. It also causes your substrate to lose it's buffering capacity faster.
Add in the amount of CO2 you are using, you are causing even wider pH swings than necessary.
Doing those two little things can help to create a more stable environment.
If you want the environment to be *MORE* stable, you need a low tech tank. And, as I said, that defeats the purpose of your setup.
If you are having a problem with algae, then you can also try raising the lights, have the lights on for less hours during the day (and/or put a "siesta" in the middle of the lights being on) or use less lights. (if you have more than one light on the aquarium)