Is a Drop checker sufficient to monitor CO2 levels? Or is it better to calculate it based on PH and ( I believe) GH using a chart since a green drop checker simply means between 15 & 30 ppm? Also, I know it is always stressed that you need 4Dh solution, does it have to be exactly that? or simply at LEAST 4 DH so that I need not worry about it if I have hard water?