I have one of my guys in a 2.5 but honestly, he is very little and has SBD. I am moving in the next few months and I think when I live alone all my fish will be more spoiled but I can't do it now.
This is my tiniest tank. I know people say you can't cycle a 2.5 . . but I test the chemicals and I haven't had an ammonia reading in over a month. I use a filter with beneficial bacteria supplement, established tank filter media, and some live plants.
I mean, Nimbus is pretty little. I think he is perfectly happy until he grows more. What do you guys think? (My other bettas live in : 6.6 , 5 or a diveded 3-way 10 gallon but that 10 gallon will not be divided 3 ways forever. I will give them more room when I move.)