0.8 Gallon's marina Betta Kit.5 led bulb shelf lite. No filtration.
With this aquarium you need to do a full water change once every 2 weeks minimum. And use prime by seachem in water changes and feed your Betta once a day with Betta bio gold.
"just keep swimming"
I have had many aquariums from brackish to fresh to being a koi butler made them giant. Just got out of my brackish tank (started to leak gerr) went back to basics.
When you see signs of stress in your fish, you can then take steps to identify the source of that stress and then to resolve it before it becomes a major issue.
If you're looking for a unique way to stock your new tank, give freshwater shrimp a try.
If your fish is suffering, you may want to consider euthanasia as an option to humanely end his pain.