Tom i never claimed that PH would drop to 4.5, i have read this somewhere on this forum, if i remember correctly he said PH could drop low as 4.5 when you have 0 KH and maximum co2. i have used the maximum co2 on that tank in the video, when i say maximum i am talking about when co2 refuses to diffuse in the reactor anymore.
Sorry if I implied you said that, it's a long quoted myth around the planted web that there's a pH drop that gets to these levels. I think maybe in the 200-400 ppm perhaps, but degassing really takes off and makes it real tough to increase the CO2 concentration.
I've never been able to get zero KH even in RO.
I suppose with high grade stuff, you might get under 5 uS.
Still, it's tough to do and keep and adding the water to a planted tank with ferts, plants, sediment, bioflims etc, I do not think it matters so muchy if there's KH or not, the CO2 might be able to be measured if the KH is really zero(scale drops off to infinity) using the pH/KH relationship.
But the rates of degassing and the CO2 content is still stable.
You can add KH and then remove it again and keep the same rates of degassing and ppm in the water. So you could measure it indirectly.
Still, these ppm's are long long way from what we can really add without toasting fish and shrimp.
Discus have been the most wimpy of fish for CO2. 40-45 ppm and a temp of 84F and O2 at 7 ppm. More than this, they really do not like it.
Smaller tetras and fish? They are pretty tough customers.
Lower temps? Even better.
The folks who seem to have the most issues with the pH/KH table being way off are mostly in the mid west and where they get the water from karst limestone aquifers, or do partial lime softening. Water companies add various products to maintain the alkalinity and those seem to cause errors in the KH and/or pH.
Places with good high grade low KH tap, they really will not have much error.
Peat and ADA soils etc, likely potting soils also since they often have ample peat, will depress the pH, causing errors, mostly causing folks to think they have more CO2 than they really do.
This is more for the general discussion, not because you or any one in particular does not know this stuff. Good to think about it and be aware and see there are some methods to measure it even if you have no KH or have tannins etc that mess with the CO2 table.
There are ways around it.
I did this using a lot of water changes when I use to use peat and wanted to know what the CO2 was. But the tap water had phosphates also, so that threw it off and I thought I had more cO2 than I really did. I was like, "hey! I have 32 ppm of cO2 and I'm not adding any CO2 gas!".
That did not make any sense either.
Rules out the peat though.
Then I got an RO unit and figured out the tap was messing with me.
Several steps, but......got the answers at least. I was running 40ppm back in 1996, but it said I had 80-90 ppm. Peat added 15 ppm of that. Then the remainder was the tap additives. Less than 1/2 was actually CO2.