I also have a (well, two!) 5.5 gallon betta tanks. They started to get some algae, and I wondered the same thing you did -- maybe I should get some algae eaters? I also don't have quite enough algae to feed the algae eater, and adding more food to feed something in the small tank would add to the potential fuel for algae too, it seems.
I got a ghost shrimp (I have read here that some bettas bother ghost shrimp; mine didn't.) I know ghost shrimp are not really algae eaters. They may snack on it occasionally, but you can't rely on them to control it. There is another
FW shrimp, an amano shrimp, that is reported to eat hair algae. Since I don't have that type of algae, and also since the ghost died, I decided not to get another shrimp.
An otocinclus may eat algae, but I don't know how well they do in a small tank, or with a betta. If they need to be in a school, the tank could get overcrowded. I decided not to stress the betta with another tankmate.
I just tried to combat the algae, and not get something to eat it after it grows. I decreased the time that my light is on, and added a few low-light plants -- java moss and dwarf sagittaria. The plants will use the excess nutrients before the algae can get to it. They do fine with the low light, and I only turn on the aquarium light for 6-8 hours a day. The rest of the time, the room light is enough.
Another thing I thought of is that my tank doesn't get much water movement. I have sponge filters in each tank, run through a gang valve to eliminate current. The still water may get more algae than a tank that has more water movement. So I guess I will always have some algae -- I need to wipe off the airline to the sponge filter every week -- but with the plants and small amounts of low light, the algae is very manageable.
