Thank you for your reply, Naty. I have mystery snails. I confused Alk & GH. L'sigh. So much to remember. Thank you for clarifying. I am thinking about just adding Equilibrium to bring my minerals up. I think my plants are short on potassium also. My KH & Carbonate are both 40 ppm.
Equilibrium should add plenty of potassium. The pH sounds decent for snails and if the diet provides good calcium and other minerals they should be good.
Does the KH test actually measure exclusively the carbonates and bicarbonates, or does it measure Alkalinity as a whole?
Far as I can tell all KH hobby tests are actual alkalinity tests. For the wider hobby out there the difference between the 2 would be very small. Like Naty said the main contributors in most tap water would be bicarbonate and carbonate (depending on the pH).
Any help with this? I feel like I got myself confused just trying to write the question.
For reference, here is the original post and chart :
https://barrreport.com/threads/co2-ph-kh-table.10717/
What one would typically do is test the KH of the water, test the pH of the water and then go to the chart to see what the estimated CO2 concentration is.
Your thinking is correct, as there may be something else affecting the KH (alkalinity) you would get a value higher than just the concentration of (bi)carbonates. This results in you going down the chart for the same pH and thus an overestimation of the CO2 concentration. Tom Barr says the same thing, here is a quote from here in that thread:
Warning, KH may not be entirely carbonate hardness. This means you will think and believe you have MORE than you actually do, thus you may be under dosing CO2.
This issues will never be the reverse, eg, you are adding more CO2 than you think.
So the error is always on the safe side using this method.
I would add that there are other things lowering the pH value besides carbonic acid (part of the total dissolved CO2). This also leads to an overestimation. For example, an aquarium with a KH of 6 may have a pH of 7 without adding CO2, just from weak organic acids such as tannins. That doesn't mean the CO2 concentration is anywhere near 18ppm CO2. The aquarium will still be ~at equilibrium with the atmosphere. I've had many tanks where the value of CO2 from the chart would be in the hundreds of ppm CO2 but the fish were fine and the indicator only a pale green. I suspect the lowering of pH from other sources (in that case aquasoil) was more to blame than a deviation from the true KH value.
The point is that the chart may not be reliable in an aquarium env. I would suggest the "pH drop method" to estimate the concentration of CO2. This works better in the medium range of bicarbonate concentrations. The "ph drop method" aims for 1 unit drop in the pH value between "degassed" water and the target.
"Degassed water" is water at equilibrium with the atmosphere. Take a small amount of aquarium water in a clean glass and allow it to reach equilibrium with the atmosphere in a well ventilated room. Measure the pH. This will be the pH of water having about 3ppm CO2. Then adjust the CO2 injection so that the pH of the aquarium water is 1 pH unit lower than the one for the "degassed" water. This will be about 30ppm for most medium KH values. You can follow it on the chart. This method aims to reduce the impact of other things on the aquarium's water pH. It is however still an estimation.
For example, aquarium water with a KH of 5 has a pH of 7 at 6am ( from accumulated CO2 and other stuff). I leave it to 'degas' and reach a pH of 7.4 ( from ~3ppm CO2 and other stuff). I then adjust the CO2 of my tank so that I reach a pH of 6.4 (7.4 - 1 = 6.4) around the time the lights turn on to have around 30ppm CO2.
Please as always, when you increase the CO2 do it gradually and monitor the fish condition over the next few days.