[anonapersona]

I'm afraid you are throwing away money on these units, and I am a real fan of the Hagen system. But, at 68 gallons, I just don't think you can keep it constant.

Keep the two you have, for one day you'll need to quarantine a fish and suddenly realize that the tank is at a much lower pH than the Q tank and you'll want to ease that stress a bit. [ quickly adding] However, I have to admit that I do not know if a different pH is actually a stress on a fish. High CO2 in a tank without plants might actually be worse. I go down the middle, with a HOB filter with high agitation and a couple of ferns and a Hagen and do lots of waterchanges anyway.