I would say an apple snail would be good enough for that size, but if you are having an algae bloom, then it's better to treat the algae based on its type. There are other algae eater such as ottos that can clean up algae, but I think you need a school of them. I am not sure if there are any other "suitable" fish for that size of tank.
You could do nerite snails or some shrimp. I second the above as well but otos are sometimes difficult to feed once the algae is under control. I think snails do a great job.
I've got a bristlenosed plec in my 10G and he does an awesome job at getting rid of algae. Didn't really see him at all for the 1st week but now hes a lot braver!