Tetra test pH is a measure of how acidic aquarium water is. Fish do not like fluctuating pH levels, and some need to be kept at specific pH levels. How it Works The TetraTest pH kit is a liquid kit, which works by changing the colour of a water sample in relation to the pH level. You simply need to compare the colour of the sample to the enclosed colour chart, to determine the pH. You will get roughly 25 tests from each kit How to Use In order to check the pH level, thoroughly rinse the test vial out under the tap, and allow it to dry. Then; 1. Fill the vial to the 5ml mark; 2. Hold the pH reagent bottle upside down and add seven drops to the vial; 3. Close the cover and shake the vial; 4. Compare the colour of the liquid to the colour chart to obtain pH value. Always read instructions carefully before use.