Blennies

• A blenoid is known for its bright colors and hardiness.Jupiterimages/Photos.com/Getty Images
  
  Blennies, which are often referred to as blenoids by scientists to avoid confusion with freshwater or brackish fish, are small marine fish with eel-like bodies and blunt heads. They skim along the bottom of aquariums and wedge themselves among rocks and corals. Blennies are omnivorous, meaning that along with red and green algae they will also pick at small copepods and other meaty foods. They are popular among aquarists for their bright colors and hardiness.
  
  

Sea Hares

• Sea hares come in dozens of varieties, but some are poisonous.Jupiterimages/Photos.com/Getty Images
  
  Sea hares, which are more commonly known as sea slugs, are slimy, slug-like creatures that range in color and size. Most of them are appropriate for marine aquariums, but you should research the specific type before deciding to add it to your tank. A universal feature of slugs is their voracious appetite for all sorts of algae. When placed in an aquarium, most sea hares will immediately take to devouring most of the algae in the tank.
  
  

Sea Urchin

• A sea urchin should be handeled carefully to avoid the prickling spines.Jupiterimages/Photos.com/Getty Images
  
  Sea urchins are echinoderms with spiky, calcium-based outer skeletons that move along the bottom of aquariums with dozens of tubed feet. They often grow to around 4 inches in length and feed primarily on algae. They use a spade-like mouth below their bodies to scrape algae off of hard surfaces in your aquarium. However, they will occasionally dine on detritus or other vegetative matter like lettuce.
  
  

Marine Crab

• Marine crabs come in several different varieties, though most are fairly small, benthic, and will spend most of their time wedged between rocks or corals. Marine crabs can be aggressively territorial by nature, so they may attack small fish if they come too close. Crabs are also mostly carnivorous, preferring meaty foods or flake foods, and will only occasionally dine on algae. According to Fish Channel, a website that provides information on fish, the emerald crab is especially prone to eating algae.