pH Checker works with any size aquarium. Holding the funnel side of pH Checker upward fill the small bulb with 20 drops of indicator solution. Slowly turn the funnel side down avoiding any spillage of the solution. Insert pH Checker into aquarium funnel side down trapping air in the funnel. Attach the pH Checker in aquarium at the furthest point from any diffuser. 

The color of the indicator solution will change yellow to green to blue according to the pH of the water. A more yellow color indicates lower pH / more CO2. A more blue color indicates higher pH / less CO2.