Aquarium Reactors are additional pieces of aquarium equipment that assist in removing high levels of phosphates, silicates, nitrates and algae. Aquarium reactors are available in many different brands, sizes and accept different types of media. They normally sit inside a sump style filter and are mostly used in SPS coral aquariums. Reactors are also useful in saltwater systems as well as freshwater. Once you've identified the issue you're trying to address, select the reactor that best fits that need.