What is the best way to clean algae in an aquarium?

My son has an aquarium with only an African Cichlid in it as they are an aggressive fish and do not like to share their habitat. He cleans the tank once a month as the pet store directed but he has an awful build-up of algae in that time. The ornaments in the aquarium are almost black with algae and are very difficult to clean with a toothbrush as the paint chips off if you scrub too diligently. We tried a snail but the fish killed him. Any suggestions? Thank you.