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hoppycalif 10-12-2006 03:05 PM

DIY Drop Checker
It is quite easy to make your own "Drop Checker" or CO2 Indicator from acrylic sheet. The object is to have a small device that holds a few ml of water/indicator solution under the water line of the tank, so there is an air gap between the tank water and the indicator solution water. The commercial versions of this range from the elegant ADA glass unit, which is currently out of stock, to a much cheaper imitation ADA glass unit from Hong Kong, to a Red Sea plastic unit which is still cheaper. All do the same job. Two of the commercial versions are the imiitation ADA unit:

and the Red Sea unit:

Both are available on ebay at: eBay: Type2 Co2 Drop Checker-monitoring proper dosage of CO2 (item 250038130859 end time Oct-18-06 09:05:18 PDT)
and eBay: CO2 Indicator Red Sea Real Time CO2 Monitor (item 300036151186 end time Oct-14-06 09:48:10 PDT)

The easiest way to DIY this is to use all straight lines and rectangles, and make it from acrylic plastic. I made one a couple of years ago, but hadn't figured out how to effectively use it so I tossed it. Here is what it looks like:

All of the pieces of acrylic have to have squared edges, and the pieces that establish the thickness of the device should be cut from a constant width strip. When glueing these together, remember, the assembly has to be air and water tight, and any smearing of the cement makes it hard to see the color of the indicator solution. For an indicator solution you can use the solution from an Aquarium Pharmaceuticals pH test kit, or any other test kit that gives yellow at pH=6 and blue at pH=7.2 - use at least double the number of drops of indicator solution as the kit says to use, to get a more intense color. (Using even 4X the recommended number of drops doesn't change the test reading, only the intensity of the color.) To use this see:

elaphe 10-12-2006 05:35 PM


Thanks for all the reviews and being a guinea pig for this.

I couldn't resist anymore. I had to order one of the ADA knockoffs from Ebay.

One question. Does the dKH of the indicator solution need to be 4 dKH or should it be whatever the tank water is?

Thanks again,

hoppycalif 10-12-2006 06:56 PM

The reason for setting the KH of the indicator solution to 4 is so that with 30 ppm of CO2 the indicator color will be green, and an unequivocal green. That gives the maximum accuracy. If you had a lower KH, the 30ppm color would be near yellow, almost impossible to judge, and if you had a higher KH, the color would be blue green, again very hard to judge. Actually, any KH that is within .5 of 4.0 will work, but 4 seems to be just about the perfect KH. Since it is easy to get the KH to whatever you want it to be, as accurate as you want it to be, why not shoot for 4.00 KH? (You can even use 10X the usual water sample size, so that each drop of KH solution will equal .1 degrees of KH. I used 4X and could judge it even closer by noting how nearly each drop came to tipping the color over to yellow. (I use AP test kits)

elaphe 10-13-2006 02:56 PM


Thanks for clearing that up. This seems like such a handy device. I can't wait to get it in the mail!

Thanks again,

hoppycalif 10-15-2006 10:42 AM

Yesterday I cleaned up the GDA, changed about 90% of the water, tossed most of the BBA infested plants, bleach dipped the anubias and java ferns, etc. and I took another photo of my "drop checker" in action:

Notice that it is running a good green color, meaning I have 30 ppm of CO2 in the water, early in the morning. I love that little bit of glass!!

quatermass 10-15-2006 12:12 PM

My glass Drop checker from Aqua Essentials in the UK (Aquili Permanent C02 test at 13) goes green when its around 15ppm.

Aquili Permanent C02 test [25265] - 12.99 : Aqua Essentials

Isn't 15ppm the right amount of CO2?

epicfish 10-15-2006 12:47 PM

30 ppm = optimal.

hoppycalif 10-15-2006 12:59 PM

It is hard to know what the best ppm of CO2 is, since the numbers people use are all based on measuring pH and KH of the tank water, which is not an accurate method, due to the other things besides CO2 that affect acidity and alkalinity in our tank water. You can choose the ppm of CO2 you want the indicator to be green at, by adjusting the KH of the distilled water in the bulb. I picked 30 ppm believing that to be at least not too much. For that I used KH=4.0 in the bulb. I am thinking about raising the KH to maybe 5 later, to get green at 40 ppm, and if that is still ok, going to KH of 6 to get green at 45 ppm. This would be a way to finally determine what the "best" ppm of CO2 really is.

Gatorguy 10-18-2006 06:28 PM

So what are you doing to make the "solution" 4 dKH?

So you want the "solution" in the drop checker to be 4 dKH, then you add a pH regeant so that you can "see" the pH change in the solution as it is exposed to more or less CO2 that is diffused from the aquarium through the air gap and into the solution?

I must be missing something. 4 dKH and "green" (my test kit is pH 7 at green) would only be 12ppm. Where am I going wrong?

I'm probably fixing to order one of those drop checkers from eBay. Why not?

hoppycalif 10-18-2006 09:18 PM

Here are the color charts for the three commonly used pH test kit reagents. For the top one, it is yellow at pH of 6.0, blue at 7.2, and green (the middle) at 6.6.

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