Siamese algae eaters aren't really suitable for a tank that's not at least 4 feet long. A 20G Long is only about 3 feet long, so its a bit small for them, especially as they grow. Ideally, they should grow up to 6 inches long each. One of the real reasons for this requirement, is that they are a shoaling species, and should be kept in groups of 6 or so.