Water fleas are small, aquatic crustaceans. They live in various environments from swamps to freshwater lakes, ponds, streams, and rivers. The most common water flea is the genus Daphnia. The water flea is an insect which gets its names because of the way it swims. They are usually seen in the spring. Water fleas are an indicator species, particularly Daphnia Pulex. It is one of the organisms used in biological toxicity testing. Therefore when you see water fleas in your system, it is an indication that your system is very clean and not harmful to the environment because they are very sensitive to toxins such as bleaching agents, metal ions, and many others.