It is neither true nor false. Amano does not use a drop checker, nor does Jeff Senske - I asked him about this before I started my experimenting with the device. So, as far as ADA is concerned, this is just a way to measure pH of the tank water. But, if you use distilled or DI water in the device, instead of tank water, and if you adjust the KH of that distilled water to 4 dKH, the water will be green when the drop checker water is at a pH of 6.6. And a pH of 6.6 with a KH or 4, with nothing in the water to affect acidity or alkalinity except CO2 and carbonates or bicarbonates, means the water has 30 ppm of CO2 in it. The air gap between the water in the device and the tank water makes the ppm of CO2 in both bodies of water be the same. So, the drop checker is measuring the amount of CO2 in the tank.

The only company that recognizes the above, as far as I can tell, is Dennerle, who sells their drop checker with the 4 dKH fluid and pH indicator reagent already in it.

The accuracy of the drop checker is very good compared to every other way of determining how much CO2 is in the water. Consider: you can tell the difference in the color between a green that is slightly yellow (pH=6.5) and a green that is slightly blue (ph=6.7). You can set the KH to 4 +/- about .2 very easily. So the range of values for ppm of CO2 when you think it is green is from about 25 to 40 ppm. Both extremes are acceptable for the plants and the fish.

I have left the fluid in mine for 3 weeks and longer without seeing any problems, other than that the glass slowly builds up biofilm, so it needs cleaning. So, if you did routine solution changes every 2 weeks you should have no problems with it.