CHEAPER HERE
When measuring CO2 ppm there are various methods one of the simples known is the measure the Ph and KH of your tank water and the correlation gives you a CO2ppm. This method is known to be flawed mainly due to the inaccuracy of most hobby test kits, therefore unless you have calibrated laboratory grade test facilities then this method can lead you to beleive you have too much CO2 or too little and then the user starts to get algae as they are chasing the pot of gold at the end of the rainbow rather than going straight to Fort Knox!!
A drop checker is much more accurate in that the 4dKH water you buy has a known KH. Therefore when the Ph within the 4dKH rises or drops You know from the change how much CO2ppm there is.
The usefullness of the drop checker is vastly underestimated because not only is it more accurate than the user testing, it is also much more convenient to take 1 second to look at it rather than do the tests. Also you can look as many times as you want through the day and I doubt anyone would want to test daily yet alone many times a day. Every time you walk past the tank you have an instant indicator if something changes.
So in summary: Its more accurate, Its more convenient and is instantly visible.
The downside to a drop checker (which is small) is that the drop checker will be 2-4 hours behind the tank water as it relies on gaseous exchange in that the drop checker is designed to trap air within it so that CO2 (and other gasses) from the tank water will leave the tank water at the drop checkers opening (much like they pass into the air at the water's surface) and then they will enter the trapped air within the drop checker. This can then change the Ph of the 4dKH giving you the visual indicator.
In conclusion your eyes are the best CO2 test in that you can see easily blue = too low, green = OK, yellow = too high + if you see fish gasping at the surface = too high, if you see lots of algae in the tank and you have dosed enough ferts and have enough light = too low.
SO there we have it. No need to test for anything. While you are watching your pride and Joy (The tank and its contents) you are visually seeing the test results in behaviour of fish and growth of plants/algae.
Andy
, that really cleared that up for me. Now to peruade my parents to let me buy this of ebay. I dont think we have an ebay account 
