I appreciate all this! I very much enjoy fishkeeping without the chemistry but in this case I want to know more as my fish's lives depend on it. I feel the need to have some basis to make decisions about levels of CO2.
I guess the thing I still don't get is the supposed "magic of -1pH" that the 2HR Aquarist and others promote. To me that is a very BROAD rule of thumb and in fact most people may not want to target exactly the "magical -1pH". If my KH is high it sounds like I may not want to drop -1pH, I might want to drop -.5pH for example, to get to my target CO2 PPM. That's why I was trying to use the chart and started this thread, was to try to figure this out. Of course one of the base problems is I chose an incorrectly labeled chart to start off with ... oops.
The other thing that is confusing is that people say, as you did in your response, "if your KH is x and your pH is y, then your CO2 is z". That's only the case if you've added CO2! If that is your water parameters out of tank/tap with no CO2 augmentation, then there is only 2-3PPM CO2. I think when people say that (including your comments above), they do mean AFTER CO2 has been added. The 2HR Aquarist has an article on that that very thing posted on this thread (subject of "wrong way to read the drop checker chart").
Thanks for the replied, its starting to sink in!
Hi! I had made this long reply on your other thread when you asked a similar question about "buffering" until I realized my reply didn't really address it and thus, was scrapped.. and I had the same problem sort of understanding it altogether until reading some of yours and @Darkblade
's posts. Now I had to rewrite it as it seemed more apt for this thread. You see, my original reply revolved around the chart itself and it might just address your questions better here as well. Let's see if I can remember most of what I wrote and along the way, clear up my own interpretations of it... and anyone else reading, please correct me if I'm wrong, I will gladly edit this post with credit.
First of all, let's clarify the most blatant error on the earlier chart with one from a better source:
and the accompanying thread: https://barrreport.com/threads/co2-ph-kh-table.10717/
and the first post by Tom:
"Measured your KH, then see how much you need to reduce the pH to get your target CO2 ppm.
Say you tap water is a KH of 5m say you want 35-40ppm of CO2, you should add enough to get the pH to 6.6 and be able to keep it there.
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 usign this method.
As the KH in your tap drops, say your KH is 1-2 degrees, there's just not much room for other sources of KH other than carbonate, at 4-5 and above, there may be.
So assuming most of the KH is carbonate hardness for a KH or 1 degree is likely okay.
As you so importantly pointed out, the intention is to pull numbers AFTER
co2 injection!! Now, I read his quote to mean if your KH is 5 and you add CO2
to get your ph to 6.6, you "have enough". Then, he turns around and says if the KH is other than carbonate, you will 'think' that you have more than you do. What's not said here is if .. you think this way, your starting ph MUST have been 7.6 .. because if you follow the chart, 7.6 is where there is properly only ~3ppm of CO2 as you yourself has noted is how you're supposed to approach the chart BEFORE injection!
[I must admit for this part, I'm a little confused by what he means about 'carbonate'. What are the other sources of KH? Someone feel free to help me out here.]
Keep in mind, this scenario works IF only you had water that somehow isolated from that "other stuff." Oh wait.. you do! I think this is how drop checkers are supposed to work. Isolate 4dKH of water made from distilled water and baking soda -- from the excellent Darkblade's Primer to planted tanks
. They fail in other ways, but that's a discussion for another post. Otherwise, like a lot of people's tanks in the world, if the starting ph is off, if your kh is a bunch of stuff, your reading will be incorrect via the chart. In this way, the chart has been more a source of confusion than help.
So, whenever you read this chart, you have to sort of 'shift' the reading -- which is what Dennis really suggests if you reread his page.
"In the example below, if I think the pH of my tank (before CO2 injections starts) and I get a reading of pH of 7 and KH of 6, the table states that I have 18ppm of CO2, (red arrow). I know that it's a false value caused by other components in the tank that contributed to tank acidity as a tank without CO2 injection will always be near atmospheric equilibrium (2-3ppm). This is a false positive. If I take a pH reading later on while CO2 is on, I need to factor this in."
Because you're "factoring this in" and making the assumption now that there are "other components in the tank that contributed to tank acidity," and as you said, you HAVE to start at 2-3 ppm on the chart, you need to ignore the ph reading at the top for a moment. Then, at your KH level, shift the whole chart left or move yourself to the right .. until you get to 3ppm. What do you start to note? at every line, from ~3 ppm to ~30ppm is a -1ph drop. 6Kh is 7.8 to 6.8. 5Kh is 7.7 to 6.7 and so on. See? Magic! -1ph no matter what KH it is. By reading the chart this way, you're attempting to mitigate the effects of the contaminants on the chart.
OH, what about the part where you asked
tank___starting-pH___CO2 required to lower -1pH (ppm)
and assuming the difference of pH in A and B is due to KH, its the relation that y > x
Wait, what are you doing here. You're not guaranteeing your starting ph is at the right point of the chart where your CO2 is at 3ppm. OR, if you are, you're not considering your ending-pH. Because if you did, why would y > x?
or similarly, this part:
I guess the thing I still don't get is the supposed "magic of -1pH" that the 2HR Aquarist and others promote. To me that is a very BROAD rule of thumb and in fact most people may not want to target exactly the "magical -1pH". If my KH is high it sounds like I may not want to drop -1pH, I might want to drop -.5pH for example, to get to my target CO2 PPM. That's why I was trying to use the chart and started this thread, was to try to figure this out.
I think the confusion arises because, against what you realized, you're attempting to read the chart PRE-injection. So, if your KH is high already pre-injection, you need to alter your start point to where it says 2-3ppm co2 and start there and get your -1pH. So yea, solely based on the chart, you still want your -1ph in high KH. Of course that statement goes out the window too if you have "other things" lowering your ph. See why looking at the chart in absolutes is pretty flawed?
Additionally, the -1ph idea AND the chart works because it's different ways to look at a formula possibly close to this one that I dug up from this post waaay back in time..
CO2 = 12.839 * dKH * 10^(6.37 - pH)
run the numbers.. I'll wait
Every slot on the chart roughly corresponds to that. As I don't know the EXACT formula Tom used to make the chart, I suppose the numbers don't match perfectly.
But what does the formula also force us to do? It makes us realize the numbers are ONLY for pH in relation to what the CO2 is doing. Given a known CO2 starting point (equilibrium), a known KH, you get a certain pH. The formula doesn't have a parameter of pH from something else not caused by CO2! It locks us back to the 3ppm starting point and the relevant ph starting point.
This thread goes into more discussions along with a similar - even more simplified formula in post #4, but regardless of how people say it, it's important to interpret the chart as simply one really inaccurate way to sort of get you in the "ballpark" of the right co2 in your tank. As Dennis and most of the old hands say, watch your plants, and definitely watch your fish as that is your priority. Wrong CO2 shows up pretty fast in fish. Wrong CO2 in plants, in time, you will start to notice the nuances -- just don't confuse them for dosing issues!