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Discussion Starter · #1 ·
I have a drop checker but currently don't have a 4dKH solution. I am pumping about 4 bps to my tank. I am using Ista Max CO2 reactor (installed externally), supposedly the most efficient way to dissolve CO2. I have a 40g tank with a heavily overstocked tank in plants.

In the day, my pH is around 6.25 to 6.5. At night when the CO2 is off, the pH is 7. During the day, my CO2 level is at least 30 ppm with a -1 pH swing? My KH is 3. My GH is 5. I do add Seachem Equilibrium to boost my GH as well as my source of micro-nutrients. I also have 20+ Amano shrimps and couple of Red Cherry shrimps. They all seem fine during the day though. They come out. They hang around by the front of the tank. They climb on the plants and eating whatever is on the leaves. They molt also. Even if I have a drop checker working, the color would be yellow anyway. Even if I want to push my CO2 level higher, the drop checker just won't show it.
 

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There's two problems with using the pH/KH relationship to determine CO2.

First, it only works if carbonates are the only pH buffer in the water.

Second, the KH test is only accurate to the nearest degree. And the pH test, if your eye and test are good, are only accurate to the nearest 0.2 or so. Let's say this is what you think you have:

pH=6.4, GH=3, CO2=36ppm

But plug the test error margins into a calculator in various ways, and you get different results:

pH=6.2, GH=3.5, CO2=66ppm
pH=6.6, GH=2.5, CO2=19ppm

At this point, you only think you have 30ppm CO2. You may be close, or you may be way off. No way to know for sure until you get some 4°KH into your drop checker.

And at 30ppm, a drop checker will be lime green, not yellow. So you will be able to detect CO2 levels both above and below 30ppm with it.
 

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I think it also only works if you assume that you have 3ppm equilibrium CO2 concentration with no injection
Correct. A pH drop of 1.0 is a 10X rise in CO2 concentration. So, if it starts at 3, it ends at 30. But, it may start at 2 or even 1. I found it takes at least 2 days for water to degas to a CO2 equilibrium with the air, and I quit playing with it when the pH kept rising beyond where the CO2 level was 3 ppm. ( I did this with a dish of very low KH distilled water on the kitchen counter, with enough pH reagent mixed in so the dish was acting as a big drop checker, using my blowing into the water to get some CO2 in the water. I just watched the water change color checking it every few hours. It was fun, but wasteful of pH reagent. You also have to use 4X the 3ml of sample water when checking the KH to be reasonably sure you have about the right KH for the water. Since water evaporates, the KH of that water was slowly increasing too, making the test useless after a day or so.

Another problem is that tank water doesn't completely outgas overnight, perhaps unless you keep it really splashing in the tank. So, the early morning ppm of CO2 is unlikely to be in equilibrium with the air.
 

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Discussion Starter · #5 ·
Thanks. I feel like I have to spend money on my tank every week. Either plants or some other stuff... I just ordered some 4dKH solution off Ebay. $7 shipped. I can't believe some site would sell a bottle for $12...
 
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