I've recently been exploring the same issues you have. Here's my conclusion:
The 4dkh drop checker is nothing more than a pH indicator. If your drop checker is green with a 4 dkH solution, all that means is that your pH is between 6.6 and 6.9 (based on the pH vs dkH chart). It has nothing to do with the amount of CO2 in the tank. 4 dkH is the standard because most people's tanks without CO2 are closer to the 7.6 range. A 1 pH drop would bring you to 6.6, which is when the 4 dkH solution would turn yellow.
If your base pH when you don't have CO2 running is between these values, your indicator will always be green and it's likely that when you add significant CO2 you will run into the yellow range on the drop checker as a 1 pH drop on a 4 dkH solution indicates yellow based on the pH/dkH chart.
Therefore, as stated above, the best way to actually determine the CO2 concentration is to directly measure the pH drop. If you want the drop checker to be more accurate, you either need a lower value dkH solution that corresponds with your reduced pH level, or increase you pH, but obviously that is highly dependent on the type of livestock you have in the tank.
Also, the difference in pH levels between your two tanks is more likely to hardscape. If you've got a higher percentage of wood in the smaller tank, you will have a lower pH because wood tends to make the water more acidic. Conversely, you may have more basic elements (like limestone or crushed corals) in the larger tank that causes the pH to be higher
Hereís a bit of clarification...the ph of the drop checker is independent of the starting ph of your tank water (assuming youíre not filling it with tank water). A properly made 4 dkh indicator solution would be made with distilled water with just carbonates only added. You can make this yourself with baking soda. So there is nothing else in the drop checker to modify pH. Therefore when you add the indicator solution, at average natural atmospheric levels of co2 (between 2-4ppm) the solution will always be blue or very close to it. If itís not blue, either your solution or your indicator fluid is bad.
When you add co2 to your tank water, it off-gasses into the air inside the drop checker at the same proportion that itís dissolved into your water. The ph scale is logarithmic which means that a drop of 1 ph indicates 10x more acidic and vice versa. In your tank the general goal is to inject enough co2 to get roughly 10x more co2 than regular atmospheric dissolved rates (so, 20-40 ppm Vs. 2-4 ppm). If you record the starting ph value of your drop checker, and then put it in your tank and drop that ph value by 1, you have increased the acidity 10x from your starting point, and therefore have 10x more co2 in your tank than you have in the atmosphere. The reason people use 4dkh usually, is simply that the color change of ph solution is easy to see at that level. If it drops by 1 ph it goes from blue to green, if it drops by more than 1 it will start to go from green to yellow.
You donít need a 4dkh solution necessarily as long as you know what color you are starting with, but for instance if you pick your starting color as green, you have to shoot for yellow, and then itís going to be hard to tell if you are overshooting the mark. Or as mentioned if you go for a 5 dkh solution instead, the starting ph will be higher so it starts as a darker blue...if it then gets all the way to a lighter green or to yellow youíre way past where you should be. 4 dkh was simply picked because itís easiest to distinguish the color changes from there as it gets more acidic.
Youíll notice there is no reference to the ph of your tank water in this explanation, because it has no bearing on the drop checker value.
However you accomplish the same thing by testing your tank ph with the co2 injected and then letting it off gas and watching to see how much the ph drops. It just takes a lot longer.
As already mentioned, the ph / dkh chart when used with tank water is only accurate if you have nothing else in your tank affecting the waterís ph other than Kh, which is rarely the case. Nitric acid is produced by fish waste, tannic acid is produced by driftwood and can occur naturally in some tap water, those are two examples that render the scale fairly useless because thereís just not much room for any error there.
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