I can't really comment on CO2
levels and pH without knowing your KH
, and if you've got any DW the whole thing is bunk.
The Redsea CO2
checker seems fine... It will certainly get the job done, and the fact that it has a white background to read the color with seems nice.
As far as the plant thing goes, I advise you to take some pictures, measure your water parameters, and start a thread in the planted tank section with all the details of your setup, including light levels, tank age, plank species, stocking, substrate, and anything else you can think of.