IMO-you don't really have a big enough tank to properly keep algae eating fish. The species that do the best with most common algae would be Oto's and plecos. The oto's are schooling fish and need to be kept in schools of at least 5 or more and in a 20gal or larger heavy planted mature tank. The BN plecos really needs at least 20gal.
It is normal and expected to have some algae in a container of water-Some can help make the tank look more natural by softening edges and a sign of a healthy system in general, however, since this is a closed system we have to keep even the good algae controlled by manual removal along with our regular water changes-just part of aquarium keeping....You can control it to a degree naturally with proper lights, color temp lights, photoperiod, water changes, nutrient control to name a few.
IMO-you shouldn't get other species of livestock for an algae problem unless you like and want that species to start with-as well as able to provide for its needs. Since this is a closed system the tank will never be able to provide all the nutritional needs for algae eating species due to their specific dietary needs and they will need supplemental feeding-then this added bioload might cause even more algae and other problems.