I started keeping bettas in November of last year. went into it with no knowledge whatsoever (my first male was in a 1/2 gal cube thing with no lid). Up until very recently all my males and females were in unfiltered tanks ranging from 1/2 gal to 2 gal and they have always been happy fish, swimming, flaring, building HUGE bubblenests, eating well, etc.
I have very recently put three of the females in a 10 gal, heated and filtered. I have not noticed a lot of behavioural differences as of yet, but it has only been 5 days so far.
I did put one of my males in the 10 gal (prior to the girls going in) for a bit while i cleaned his bowl, and i am definitely getting my males out of their bowls as soon as i have another 10 gal or a divider for the 20 gal. He was so much more happy, swimming around, exploring, flaring at the bubbles coming out of the filter output. I felt so bad putting him back in his bowl after it was clean..... :-(
Even though i barely have space for the tanks i want to set up, i am ensuring all my finkids will have the space, the filtration, the heater, etc. Just seeing Fyshee so happy for those few brief moments has convinced me.
to the last poster: yes, sometimes it does boil down to genetics (the swim disability), but the fin rot was most likely preventable in the first place.