You can only keep one Siamese algae eater in a 20-gallon tank. Siamese algae eaters are larger than most freshwater fish and can grow up to be six inches in length. Moreover, they’re also very energetic and adventurous.
For these reasons, Siamese algae eaters require plenty of space to thrive. A 20-gallon tank is a bare minimum for a single Siamese algae eater.
However, if you wish to keep more than one Siamese algae eater, make sure to provide at least 10-gallons of water for each additional fish. With that in mind, a small school of five or six Siamese algae eaters will need at least a 75-gallon tank to live comfortably.