I have pretty good luck with my betta's so maybe I can help.
I started with 5 gal tanks and they were tough! Not big enough for a decent filter, so water changes had to be done daily. My boys got very unhappy if I skipped a day and took out more than 50% the following day.
I have since switched to 10 gals with dividers and just do weekly water changes as with my other tanks. I use the small AC hob filters and they do well.
I do have a tendancy to over feed because they almost eat out of my hand. I feed them the betta pellets from Ken's and give them each 3 twice a day.
Good luck!
Thanks for your help! Boy, I guess the 5 gallons are tough! I should switch to a 10, but don't have a place to put it. for now I have to figure out how to keep the 5 gallon happy.