Oh, so you are adding a gh booster. Thanks for the info!
Weird on the Oase 600. I have 2 for my upcoming UNS 120U build. While I haven't run with plants yet, I have filled it with water and run the Oase 600's and it didn't seem like they were lacking for power? At 435 liters I would say 1 Oase 600 is not quite enough for high light/co2/stem plants but just barely. But that's my very subjective, not quite setup observation, lol! Other's may have a different take on that.
I have been using the Aquario Neo diffuser's for the past 2 years. Very nice micro bubble creation and I don't really clean them much but I have a compartment in the back of my hi-tech tank that all the equipment hides in. They have a new version these days that is sleeker but it will still stick out like a sore thumb in a tank without a back compartment as most in tank diffusers do. The only mist I get is from pearling. But that's on a 10gal tank.
And this has me thinking... Any co2 check valves in the mix?
If so, I replaced my plastic one that came with my co2 unit with a Nikou stainless steel check valve and my co2 fluctuations that I had been battling the past couple of months went away. So maybe the limiting factor is the check valve? Not allowing enough pressure thru to allow the inline diffuser to work properly? Just a thought as most inline diffusers require a pretty hefty psi to operate effectively.