You know, your fish needs more room than that. I don't care what the petshops or manufacturing companies tell you, a beta should have AT LEAST a gallon. How would you like to spend your life in a bowl barely bigger than you?
Goldfish (the cheap ones/aka fair fish) should have a filter, a heater, and at least a 20 gallon tank...the more expensive ones should have the same and at least a 10 gallon tank.
Sorry, I know he was a gift and he's a beautiful beta. Just please PLEASE don't leave him in there or else he won't live a very long healthy life.
Love the shirt btw, I have one too

he himself wasn't a gift. i've had him for almost three months now in a bowl that same size. i don't keep goldfish, so don't worry about that. i didn't even get aristotle until several weeks after my friend bought their own betta - which lived perfectly fine in a bowl the same size - because i was worried that i wouldn't be able to manage it. when i got him, she switched her fish to a three gallon tank (after taking all of the necessary precautions, mind), and it soon died; none of the subsequent fish have lasted more than two weeks. i've no idea if that correlates to aristotle's situation, but he's had no complaints.
it's certainly bigger than the container that he came in, any way (which was literally no bigger than he is), and i guess they live several weeks in those with no problems.
i appreciate your concern for my fish's wellbeing, but i'm pretty sure he'll be fine. he has been so far, at least.
i'm not sure i understand your purpose in posting that.