I'll be the odd one out. I have a betta in a 1 gallon bowl. He is very happy and healthy, in much better shape than he was in before. I have a 2.5 gallon jug that has 1/2 dechlorinated tap and 1/2 ro water, since Bettas like soft/acidic water in nature. The 1 gal bowl has gravel with some shiny stones for aesthetics. My secret? I scoop him out with the cup that I bought him in and put it aside, then I dump all his old water out from the bowl, wash and rinse the crap out of it multiple times, then refill it with my water mix. And just to be safe, I dechlorinate the water again with a special betta kind ($4 at petco). I then dump the water in the cup into the sink, along with the betta and catch him with a net and put him in his fresh and clean water. When I first got him, he had ammonia burns, I used this method and added a little seachem stress guard every week for about 3 weeks and he just sprung right back. I have had him for many months with zero issues. Just remember to feed him a tiny amount, once daily, I treat him with a little brine shrimp occasionally, and it's ok to skip a day on feeding. Keep in mind, it's nearly impossible if not completely impossible to have a fully cycled 1 gallon betta bowl without a filter (which I don't). Which is why I do 100% water changes every week. Doing it this way, doesn't give the ammonia a chance to hurt your betta, so it works. I know people that do it every other week and it works for them, but I recommend a 100% weekly and you'll be fine.