My recommendation comes from owning and watching my own fish. They use every inch of my 75g but I think a single (probably stunted) specimen would do ok in a 2ft aquarium if it was as beautifully planted as your bowl.
Possibly the most respected source of caresheets about atm and that suggests a minimum of 30 inches. Hes a fish, he doesn't need human companionship but he does need space to hunt, swim, nest and do everything else that comes naturally. From your photos he looks very pale, as the only occupant of the tank he should have strong dark colours to proclaim his health and dominance. He looks like one of my ladies when they have just lost a fight and are signalling their submission. Its a very clear sign of stress and has alerted me to problems in the past.
The recommendation you site is for a pair
of fish, a fish that is notoriously territorial. I don't see how that applies to my case. I also don't what "clear sign of stress" you see but his color and activity are fine so your going to be more clear.
As far as sources for tank size I find this white paper
written by University of Michigan professors to be much more applicable. The paper is a study of early and adult social behavior in paradise fish. If you read you will see that they use tanks ranging from .5 gallons, 2.3 gallons, 5 gallons and 10 gallons to conduct their experiments.