Bromothymol blue

Bromothymol blue is a chemical pH indicator liquid used in aquariums to test the pH of water or for CO2 testing by aquatists who keep planted aquariums.

How is it used?
A few drops are placed in a solution of about 1ml of pure water with a reference level of 4dKH. Aquarists either make this up themselves or they purchase the liquid. The liquid is placed in a small glass U-shaped open ended cylinder and placed inverted into the aquarium water. After about an hour or two the CO2 in the aquarium water will dissipate into the test liquid and alter its pH level therefore making the Bromothymol blue turn colour. 

It is also used by various manufacturers selling pH test kits for testing the pH of water.

Colours

 * Green for pH 6.6
 * Yellow for pH < 6
 * Blue for pH > 7.6

Links

 * See CO2 article
 * Bromothymol blue on Wikipedia
 * Shops selling BB - Aqua Essentials