Using a Drop Checker/CO2 IndicatorWhy do so?
Look back thru the posts here and on other forums and you see many cases where people report having 100-200 ppm of CO2 in their tanks, with no fish problems. Others report they have “good CO2”, 20 or even 40 ppm, but they can’t seem to get rid of BBA. Still others have endless algae problems, but using the KH/pH measurements of their tank water they are afraid to raise the CO2 level any more because the fish will all die.
These are all signs that the KH/pH measurement method just doesn’t work for typical aquarium water. That shouldn’t be a surprise, because all of the write ups about that method, including Chuck Gadd’s, mention that it is only good if there is no phosphate or other ions in the water that affect its KH or pH, and we dose phosphates, add wood and peat which decrease pH, and who knows what else in the water is affecting the KH or pH readings.
Carbon is the primary element plants are made of. When a plant grows it has to have a supply of carbon, and that is almost always from CO2. While we can measure with considerable accuracy how much nitrogen, phosphorous, and potassium we are making available to our plants, the amount of carbon we make available is largely a guess. It makes good sense to try to know that nutrient’s concentration in the water at least as well as we know the others.
Someone determined that when they dropped the pH of the tank water by 1.0 (from 7.2 to 6.2, for example), they knew they had enough CO2 in the water, or about 30 ppm. This test was done by taking a sample of tank water, letting it sit until the CO2 in the water “degassed”, reading its pH and then subtracting 1 from that reading as a target for the tank pH. This method depends on the fact that a change of 1 in pH equals a 10 to 1 change in ppm of CO2 in the water. So, if the degassed water ended at 3 ppm of CO2 this would give you 30 ppm of CO2 in the tank. But, those of us who measured the amount of CO2 in water that sits out in the open to degas, found that the amount of CO2 in that water continued to drop for 2 or 3 days, down to as low as .5 ppm. And, a degassed ppm of .5 means the tank water would only be at 5 ppm with a pH drop of 1. So, this method doesn’t work well either.
The in-tank CO2 indicator has been available for several years, most recently as an elegant glass device sold by ADA and as a look alike from a Hong Kong dealer on ebay. But, the makers of those devices have always told us to use tank water in them, which makes them no better than just testing the tank water directly.
We need to know how much CO2 we are supplying to our tanks. If we don’t know it with some accuracy we will refuse to increase that amount for fear of fish deaths. And/or, we will spread misinformation about how much CO2 can be or should be used in aquariums.
Why does a “drop checker” work?
Imagine a jar of water, with a tight lid on it and an air space above the water. Assume the water has some CO2 in it. CO2 goes into and out of solution with water very easily, so molecules of CO2 will constantly be moving from the water to the air and back. This system must reach an equilibrium where the rate of CO2 leaving the air space is exactly the same as the rate entering the air space. If that were not true, all of the CO2 or none of the CO2 would end up in the air or the water. But, we already know the CO2 molecules easily move from air to and from water, so it can’t all end up in one or the other location.
So, with X ppm of CO2 in the water and Y ppm in the air space, the system is in equilibrium. Now, add a second blob of water connected to that same air space. The same equilibrium must be reached between the air space and the new blob of water - Y ppm in the air and X ppm in the water. So, the new blob of water must reach an equilibrium where it contains the same X ppm of CO2 as the water in the jar.
That is what a “drop checker” is - a little blob of water connected thru an air space to another bigger container of water.
How do we make a “drop checker” do what we want it to do?
First, what do we want it to do? We want it to show us that we have a certain ppm of CO2 in the water. This gets us back to the KH/pH/CO2 tables and equation. We need to eliminate the reasons why that equation and table don’t work for aquarium water. The way to do that is to use water that meets the requirements of the equation - no ions that affect the KH or the pH except CO2 and carbonates.
Distilled water has nothing dissolved in it, so no ions are there to affect either KH or pH. Deionized water also meets that criteria. So, if we use either distilled or deionized water in the drop checker the KH/pH/CO2 table works just fine. All we need to do is add some carbonates or bicarbonates to get some KH in the water, and some pH reagent so the water color will tell us its pH.
Since we want to get a clear indication that we have the right amount of CO2 in the water, we need to select a KH that will make the pH indicator give us the most accurate reading possible. If you look at the color chart for the commonly used pH reagent, Bromothymol blue, you notice that the middle of the chart is green, and a small pH change changes the color to a bluish or yellowish green. That is the easiest point on the color chart to identify. And, that occurs at a 6.6 pH. So, we want the pH to be 6.6 when the water has the desired amount of CO2 in it.
The hard part is deciding what the desired amount of CO2 is. Up until now everyone has measured their ppm of CO2 using the tank water’s KH and pH, but we know that isn’t accurate. The desired ppm has grown from 10 to 15 to 20 to 30 ppm over the past few years, based on those measurements. For now it seems prudent to assume 30 ppm is the desired amount, but in the future that number might end up being just about any number from 20 to 50 ppm.
At 30 ppm of CO2, with a pH of 6.6, the KH will be 4 dKH. So, our distilled water should be dosed with sodium bicarbonate to a KH of 4 dKH. That isn’t much bicarbonate of soda!
You can mix up a batch of 4 dKH water using distilled or DI water as follows:
- [*]Add 6 grams of bicarbonate of soda to one liter of distilled water. This gives you one liter of water with a KH of 200 dKH. Now take 10 ml of that water and mix with 490 ml of distilled water (a dilution of 1 in 50) and you get 500 ml of 4 dKH water.
[*]We don’t all have a gram scale capable of measuring 6 grams accurately. Most of us do have a KH test kit. So, it is also possible to arrive at 4 dKH water by mixing a very small amount of sodium bicarbonate into distilled water and repeatedly diluting it with more distilled water until the test kit says we have 4 dKH. Relying upon test kits isn’t a good idea, but in this case it is a good enough idea if we just want to monitor the ppm of CO2 in the water. Buying certified 4 dKH water is a better idea, and that product will very likely be available soon.
- [*]Add enough 4 dKH water to the bulb of the drop checker to fill it to where the manufacturer recommends, or half full for the ADA style glass device. A syringe works very well for this. Flushing out the bulb with some of the 4 dKH water is very desirable. Add enough pH reagent to get a strong blue color, but not enough to make the water opaque. The exact amount of reagent isn’t critical.
[*]Place the drop checker in the tank, under water, so that an air bubble is trapped in it to separate the water inside from the outside, and wait about 2 hours for it to reach equilibrium. It will be green if you have 30 (actually 25 to 40) ppm of CO2 in the water. If it is yellow-green, you have too much CO2, about 50 ppm. If it is blue green, you have too little CO2, about 20 ppm.
Another type of drop checker
This is a slow working device. It takes a couple of hours to reach equilibrium with the tank water. If that is too slow, it is possible to speed it up considerably by replacing the air space separating the tank water from the drop checker water with a gas permeable membrane. Many thin plastic or rubber materials are gas permeable. One such material, that works surprisingly well, is Tyvek, which is the material Priority Mail envelopes are made of. A small container of water capped with that material will act as a drop checker.
To achieve a rapid response requires that the volume of water in the drop checker be at a minimum and the area of the membrane that allows CO2 to pass thru be at a maximum. For a disc shaped blob of water, that means a thin disc, shaped like a coin. To further minimize the volume of water a piece of sponge can be fitted in the water chamber to displace some of the water. If the disc of water is loosely filled with sponge and is only about a sixteenth of an inch thick, the device will reach equilibrium in about a half hour. It will be much harder to read the color of the water in it, but it will work just as the air gap type drop checker works. No commercial version of this design is now available.