For NPK you buy titration test kits from your local fish shop (LFS), or from online stores like ebay (or the sponsor's websites) or you could buy electronic meters (pricey, but quick readouts). Then you add a certain number of drops to a vial of tank water and then wait a few minutes. The liquid changes color then you look at an index card that comes with the test and see which color it looks most like and that is the ppm.
The CO2 measurement is more tricky. You can get a drop checker from the sponsor's webpages (adds on APC), or perhaps ebay. Drop checkers take 4 hours to detect the CO2, and don't give you an exact ppm readout, they just tell you if the CO2 concentration is in the correct range or not. Alternatively, you can measure your KH (carbonic hardness measured with a titration kit from your LFS) and pH and then plug these numbers into an online pH-CO2 chart to figure out the approximate CO2 concentration. Scroll half way down for the calculator and chart
http://atlas.csd.net/~cgadd/aqua/art_plant_co2chart.htm
Also, make sure you have a look at the fertilator. It helps you calculate the approximate ppm that results when you add a certain amount of fertilizer.
Oh and next time don't double post
http://www.aquaticplantcentral.com/f...-fert-co2.html 