EcoBio-Block 2-Pack Stone for Aquariums, Medium - Natural Water Clarifier and Odor Remover, Treats 30 to 80 Gallons Volcanic rock containing beneficial bacteria colonies, disperse about every 30 minutes. Clarifies cloudy water and eliminates odors Speeds up nitrogen cycle, with 100% pure bacteria in spores which germinate in early, middle and late stages, only germinating in the presence of organic waste. Reduces need for frequent maintenance by continually replenishing beneficial bacteria for 1 and a half to two years Safe for all fish, reptiles, amphibians and other aquatic life Simple to use, no mess aquarium care in fresh or salt water. M size treats tanks from 30 to 80 gallons.