Well, yes and I said as much. And this isn't intended to be a t-shooting session and I'm not trying to pinpoint my CO2 saturation.
I wasn't trying to be argumentative, just making a point.........and in all fairness...................
I think what I'm aiming for is to get an absolute value for the CO2.
So that is what I was responding to.
It's a 15 mile drive one way to get my cylinder refilled and it costs $30 bucks each time. I'd really like to know I'm getting the most of each refill so I want to get a handle on efficiency. I've gone through two refills in the past two months and that seems excessive (and I've checked for leaks with soapy water)
But back to the matter at hand.
You are saying you are filling a 20lb tank once a month on a 65G tank?
If so, something is way, way off. I'm not sure what, but something is just not right for that size of tank.
For reference, I go through a 10 lb tank every 4 months with my 120G. High light, pH driven down 1.35 from degassed value. CO2 on for 8.5 hours per day.
Now my KH is less than 1.0. But back when my KH was in the teens it was about 10 weeks between fillings. So it does take more CO2 to drive down the pH of higher KH water relative to lower KH water. But I doubt tht has much to do with your situation. And the HOB filter could be a factor, but I do run pretty vigorous surface agitation all day long, so have quite a bit of off gassing as well.
I don't what is going on, or where to start other than looking for leaks again.......but like I said something seems to be way off. Maybe others have ideas??