The precise taxonomy and origin of the common "bristlenose" is unknown, and some authorities believe it may be a hybrid of other species. Several selectively bred varieties exist, including the albino and long-fin forms. Depending upon the species, sizes range from 3 to 4 inches, with a few species attaining five inches. When I authored a profile of this for another site, I said one could manage in a 10g, a pair in 20 gallons. I'm not sure I would do this, maybe. As AdoraBelle says, the bioload is an issue, much more than the size. The fish would be perfectly happy in a smaller space if it has food, but the aquarist has to ensure the water is clean. Herbivore fish like all herbivorous animals need to eat a lot of food compared to omnivores and carnivores.