I just bought an imitation ADA "drop checker" on ebay, eBay: Type1 Co2 Drop Checker-monitoring proper dosage of CO2 (item 250036844403 end time Oct-14-06 06:06:32 PDT)
and have started experimenting with it. It seems to work extremely well!

The secret to using one of these as a CO2 meter and not as a pH indicator is to use distilled water, with KH adjusted to 4.0 with baking soda, in the bulb. At a KH of 4.0, the pH that goes along with that KH when the CO2 is at 30 ppm is 6.6, and at pH of 6.6, the AP pH test kit or any other that uses that indicator, shows an easily recognizable green color. Also, slight errors in measuring KH, say +/-0.1, which is an achievable accuracy by using 4 times bigger water sample, do not affect the reading much. An error in judging the color that is well within practicability is +/- .1, and that error gives a CO2 ppm error such that the 30 ppm may be between 25 and 40 ppm, which is far better than any other method we use. This is great because it removes completely the effect of having odd tank water, with other sources of alkalinity or acidity than carbonates and CO2.

I set up my "drop checker" with KH=4.0 water, an excess of indicator reagent, which I have found has no effect on the accuracy, but makes judging the color easier, and set it in the tank about 3" below the water surface. It took about 2 - 3 hours for the color to stabilize - unfortunately at yellow! My fish were mostly at the surface gasping!

I will be continuing to use this, adjusting the bubble rate to keep the indicator color at green and will report back here later about how it is working. Right now, for the first time in months I can see "light at the end of the CO2 measurement tunnel"!

Good job VaughnH, and good thinking. It will be interesting to hear how you get on with this.

A few things occurred to me while reading you post:

I’m no expert on ph indicator solutions and don’t know how much they vary but it would perhaps be useful for anyone using this method, that has the opportunity, to use a well calibrated ph meter to check at what ph their solution becomes a particular colour. If anyone does this they could post their findings for the benefit of others.

A lot of people including my self set their maximum CO2 level (bubble rate) by careful observation of the fish. In my case I increase the CO2 slowly and carefully until the fish start showing signs of breathing faster/more heavily and then back it off a fraction so that the fish are breathing normally. Using this method and based on the ph/kh tables (using a well calibrated ph meter) my tank apparently runs at around 90ppm. I think many people using the kh/ph tables are finding that they apparently have even higher levels of CO2. While many fish can almost certainly tolerate considerable more than the often quoted 30ppm it seems likely that as VaughnH suggests the very high levels many are apparently getting is due to the effect of other acids in the water.

What I think would be interesting would be to try and establish roughly what the CO2 level is in tanks where the level is set as high as possible using observation of the fish. A possible solution here based on VaughnH’s idea would be to establish at exactly what ph the indicator becomes green. Then use a range of solutions at different kh values (e.g. 4, 5, 6 etc) prepared as suggested by VaughnH to establish what kh is needed to achieve a known ph in a tank with maximum amounts of CO2 added based on fish health and consequently the CO2 level in such a tank.

Cheers
Neil

Merely setting the KH at a referenced level, does not mean it's going to stay that way, just like the CO2 level inside the bubble.

If the CO2 concentration changes inside the bulb, what do you think the KH does inside the bulb?

You can set the KH correctly, but other organics, hydroxides and tank's KH will alter things over a day or so, maybe less depending on the tank's KH, ionic differences between the solutions.



Regards,
Tom Barr

Tom, are you saying CO2 alters the KH? There is no mixing between the solution in the "drop checker" bulb and the aquarium water, so I see no way for the KH to change unless the CO2 does it. When we do the check where we "degas" a tank water sample, then shoot for dropping the pH of that sample by 1.0 in the tank, we are assuming that the KH is not changed by adding CO2.

How do ionic differences change what is in the "drop checker"? As I understand how this little device works, the air gap between the tank water and the "drop checker" water isolates the two liquids, but CO2 reaches an equillibrium with that air and between the air and the "drop checker" water. So, the assumption is that the ppm of CO2 eventually is the same in both bodies of water.

Obviously, one day doesn't prove anything, but I don't see how, theorectically, this will not work.

I am getting confused now! I'm not saying this is an unusual condition for me, but.....

The equation for CO2 in water is: CO2(ppm)=3*KH*10exp(7-pH)

I am assuming that this equation says that if you dissolve CO2 into pure water there will be an equillibrium reached between the pH and the KH, and the equillibrium values of each depend on the ppm of CO2 dissolved. Is this correct?

Assuming it is: How can this equation work if the water isn't pure - if it has a non-zero KH when there is no CO2 dissolved in the water? The only way the equation gives a zero value for ppm of CO2 is if the KH is zero.

So, how can this equation give a meaningful answer if the water has, for example a KH of 10, due to sodium bicarbonate for example, with no dissolved CO2 in it?

My lack of knowledge in this area is annoying me!

No, I'm not saying CO2 does anything to KH.

I am saying the referenced KH you added will diffuse out, just like the CO2 diffuses in.

That's how these devices work.

Use the tank's water, not a referenced distilled water and baking soda.

The water inside these checkers will equilibrate with the tank's water.
So the KH and other things in the tank will influence the checker over a few hours/day or so etc.

The CO2 has to get in there to change the color and show that there is a change in CO2 through the day right?

Same deal here, except in their case, the issue is KH changing to what is inside the tank and diffusing out of whatever small referenced KH solution you added to the checker.

Not sure if this is clear enough.

You are not adding KH.
You are adding CO2.

The KH will change becuase of the tank's KH and other ions will diffuse into the checker. This will not take that long.

Then that reference solution in the check will show a different reading.

I think the idea is good initially, but the reality is that the the KH/checker solution will change rapidly and you may as well just add tank water, not distilled and baking soda etc.

Regards,
Tom Barr

This must be a slow brain day for me: how do carbonate ions cross the air gap to go from the tank water to the indicator water - there is no physical contact other than the air gap and the glass surface?

In a couple of days I will be satisfied with my CO2 bubble rate - I hope, and I will test this with just the 4 degree KH water in the bulb, then recheck it in a few days to see if the KH has changed. My tank water is less than 1 degree KH so the change, if any, should occur pretty quickly.

Opps, air gap, I was worried you'd say that
Then there should not be any issue in that case.
Some of the checkers come in contact with the tank water.

This will work well and is a VERY good idea in that case.
I'm the one with the slow brain day

Regards,
Tom Barr

That's a relief - I can tear up the high school enrolment papers now!

Next issue: does anyone know if the indicator (dye) fades with continuous exposure to aquarium light intensity? My indicator is a greenish golden color now - meaning about 45 ppm of CO2, but it is not intense enough to satisfy me, and I have about 12 drops of reagent in the mix. (I'm using the Aquarium Pharmaceuticals reagent instead of the tiny bottle of stuff that came with the device.) Maybe I should try the cheapy reagent next. It seems like there would be a maximum amount of reagent you could mix with the water without changing the chemistry - I think it has sodium hydroxide in it.

My little drop checker is working great now. I have adjusted the bubble rate a few times and am now keeping the checker color on green from about an hour after the CO2 comes on until it goes off. Here is a photo of it in the tank - green!



I see no signs at all of the fish being stressed by the CO2. So, my next step will be as Neil suggested - change the indicator solution to KH=5, which will make it be green at 40 ppm. Then I will watch to see if the fish have any problems at that ppm. If not, and after a week or so, I will try KH=6, which gives 45 ppm when the color is green. I keep wondering if we reallly know what the maximum CO2 concentration can be without bothering the fish, since I don't know how anyone knew what their concentration was using the methods we had to measure it. And, is there any theoretical way to determine what is too much for the fish? I do know that 70 ppm+ is too much because I had that briefly when I first started using this device and all of the fish were gulping air at the top.