Description To be used with any CO2 drop checker. The solution changes color to provide a quick and accurate indication of CO2 levels in an aquarium. This version is a concentrated solution that should be mixed with water from the aquarium to be monitored. Typically 2-4 drops per drop checker filling. Specifications Yellow: high CO2 (danger to fish) Blue: low CO2 Green: balanced - enough CO2 for plants, not too much to suffocate fish