I just went from the Red Sea Turbo co2 system to the Pro (cylinder) system two days ago on my 55 gal tank. I turn off the system at night and have the bubble counter at 30 during the day. Last night before turning the co2 off I noticed the drop checker was turning yellow so I turned on the underground air stones during the night to give the fish more oxygen. This morning I found the drop checker at yellow. I tested the KH and it was at 22, the GH at 12, and PH 7.0. What am I doing wrong and how do I fix it?
Thanks.

Something isn't being measured correctly. Do you have 4 degrees KH, distilled or DI water in the drop checker? Is the KH you mentioned degrees of KH or ppm of KH? Is the GH degrees of GH or ppm of GH? What do you mean by "have the bubble counter at 30"?

The KH/GH test kit (from API) is in degrees. The direction for KH indicates one drop is equal to 1 dkh or 17.9 ppm it took 22 drops to get the result. It took 12 drops to get the result for GH and that equals 1 degree or 17.9 ppm. I didn't use distilled water in the drop checker, the directions from Red Sea said to use tap water. I will change it to distilled. As far as the bubble counter...I have it so that there is 30 bubbles per minute. Should I pick up a different test kit. I did the test twice with pretty much the same results.

If your KH test kit is working right you have very alkaline water, but just high calcium/magnesiun concentration, and not too high at that. Did you or do you add baking soda to the tank? That would drive up the KH and not the GH. The drop checker will not give usable results unless you use distilled or deionized water in it, with a known KH due to carbonates only. The Red Sea instructions are just a way to measure the tank pH using a drop checker. And, 30 drops per minute are not going to be enough CO2 for a 55 gallon tank. Something like 3 or 4 times that amount might be enough, depending on how the CO2 is introduced and whether you are losing CO2 at the water surface due to agitation of the surface.

You can buy known KH, distilled water solutions, that will make the drop checker measure the CO2 ppm in the tank water from KH Standard

Thank you very much for your help. I replaced the tap water with distilled in the drop checker and ran air stones all day with no co2 with no change in the co2 reading. I also did a partial water change and the CO2 remains high. The fish are acting fine and not gasping for air so I think my drop checker may be off. I purchased a new KH kit and my reading was 3 degrees. So the test kit was bad. I looked at the link for the Standard, do I need the 4dKH or the 5 dKH?
Thanks again for all of your help!

If you use 4 dKH water the drop checker will be green at about 30 ppm. With 5 dKH water it will be green at about 40 ppm. I think 4 dKH is best.

Your drop checker will not work if there is just distilled water in it, which has zero KH. The water has to be adjusted to 4 dKH using a carbonate or bicarbonate compound, such as baking soda.