Most people think of algae as an unsightly nuisance in the marine aquarium. However, algae can provide several important benefits for saltwater tanks if kept at manageable levels (and preferably off the front glass). Understanding algae will help you maintain a healthy saltwater aquarium.
Many saltwater fish subsist on algae in the wild. In captivity they usually require supplemental feedings of algae-based food. However, a layer of algae on rockwork can provide ample grazing for these herbivores. Grazing like this more closely mimics their feeding behavior in the wild. Marine fish that like to graze on algae include the various surgeonfish/tangs, rabbitfish and blennies. As long as it doesn't get out of hand, a light growth of algae on your rockwork can provide food for your fish.
Algae can also provide food and shelter for beneficial organisms like copepods. Copepods provide good nutrition for saltwater fish that prey on small crustaceans. Algae growth can encourage copepods to grow and reproduce in an aquarium. Some saltwater hobbyists encourage the growth of copepods by establishing refugiums, smaller tanks that drain into the main tank. Refugiums typically contain coral rubble and algae to provide food and shelter for copepods. Some of the copepods get washed into the main aquarium, where fish can snack on them.
Most coralline algae belong to the red algae clade, but some green algae have similar properties. These algae grow slowly and have beautiful colors, mostly purples, pinks and reds, making them more desirable than most microalgae. They can also provide a hard surface on which corals can anchor themselves. Coralline algae usually arrive in a fish tank on high-quality live rock, and improve the aesthetics of the aquarium.
Most people think of algae as various types of scummy green organisms. However, seaweed is actually a type of algae called macroalgae. These large algae provide many specific benefits in the aquarium. They have an aesthetic appeal and require less care than many saltwater organisms like corals. Additionally, macroalgae will absorb nutrients from the water, improving water quality and making it harder for microalgae—the gross algae—to grow. You can also use macroalgae as a food for refugium-dwelling organisms.