Its 1 betta in a 2.5 gallon. You don't need a filter, just give him a fresh gallon once a week.
My mother has had a goldfish in a 2 quart bowl on her living room hutch for 3 years. No filter, no lights, no air stone.. a fish and a handful of rocks.
(Not saying its the best life ever, but betta's live in puddles....)