pH kH tester

[bernie]

To measure C02 concentration, I need a pH & kH tester?
Any recommendation on a reliable & affordable one?

What about gH? What does it indicate/measure?

[tawauboy]

yes. and, you'll need a ph/kh chart to determine the co2 concentration.
for ph, you can get reagent test kits from lfs or an electronic ph meter.
for kh, you can get reagent test kits from lfs.
tetratest is a widely used brand.