The myth is not the KH will buffer the water and cause less a pH swing, that is science. The myth is how this is somehow super important and we need to keep a certain amount with co2 injection
to avoid some major crash. A high KH will not significantly change the pH swing due to CO2. Injecting co2 will alter pH. It has to. Controller, solenoid, whatever, it doesnt really matter. I get a huge swing between night and day and this is fine. I set my controller so it maintains around 30ppm during the day and my tank fully degasses CO2 overnight, about a 1-point pH shift every day (I suspect this is much more than most, since I have a lot of gas exchange with overflows), and I have a KH around 2*. I would see 0 difference in night-day swing by doubling or tripling my KH, as it will be much smaller than what our tests can measure, like hundredths or thousandths(?) of a pH point less.
Lets look at an example. We will have to assume this is a perfect tank with no other acids/bases, and use chucks calculator at http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm
. KH=1, pH=7.0 with no CO2 addition (3ppm ambient). Then we add CO2 until it reaches 30ppm. According to the calc, then our pH will be at 6.0, a 1-point drop. Now lets go to the other extreme, our KH=10 and our pH=8.0 with no added CO2 (3ppm). Now we bring the CO2 = 30ppm and according to the calculator we will have a pH=7.0, still a 1-point drop. I fail to see how KH has buffered the pH swing due to CO2.
Now perhaps it is different if your dumping in stronger acids in your tank? IDK. Other than in an instance were KH is being consumed in your tank, like I said earlier, or in hard water fish, or while using RO water, I see no reason to mess with KH if you have at least .5* . IMHO having to worry about one less thing in your tank, priceless
I dont know the history behind the myth. I've only been into CO2 injection for about 3 years. But I figure it went something like this. Joe sets up a pressurized CO2 tank. After running it for a few hours he check his tank. he screams holly moley my tank is crashing my pH has droped a whole point! He dumps in enough baking soda or whatever so his pH goes back to near where it was before. He wipes the sweat of his forehead and feels confident now his pH is back were it was. But this is simply due to not understanding that CO2 drops pH with no effect to fish. I suspect those that dont have a large pH swing attribute it to higher KH but this simply is not the case. More likely they have a low amount of water turbulence (slower gas exchange) and dont gass off much CO2 overnight.