Yes, you can keep one Siamese algae eater. In fact, if you have limited space, keeping a single Siamese algae eater in a tank is recommended. This fish can grow to be six inches long, and as an energetic species, it requires plenty of space to thrive.
With that said, Siamese algae eaters are schooling fish. While keeping them in groups is unnecessary, they express their natural behaviors best when they’re in a small school of five fish or more.
Alternatively, you can keep your single Siamese algae eater in a community tank. As such a gentle species, Siamese algae eaters can get along with a wide variety of tank mates, such as gouramis, barbs, cory catfish, and nerite snails.