Saltwater aquariums typically use a macroalgae like chaetomorpha or sea lettuce to combat excess nutrients and unwanted algae growth in the display tank by growing special types of algae in a refugium. The refugium allows you to give the algae the right light for it to grow but also acts as a safe-harbor from fish that may try to eat it. With the different types of algae, a lot of reefers like to create a display like refugium that showcases the different algae instead of just letting it grow wild. If macroalgae like chaeto grow freely, it will create a ball-like shape that is easy to harvest from when the refugium gets full. Many saltwater aquarists find that with a properly lit and sized refugium, the algae can actually take in too many nutrients starving your corals which can be solved by decreasing the photo-period.