couple bits to put in
large daily oscilations of temperature, no matter how slow the change is, are bad for betta health
feed up to the volume of their eye every day
if you see surface scum, bottom floaters or mulm, especially in this kind of tanklet, full water change on the spot
chlorine+chloramine treatment is mandatory now, almost all municipality are using chloramine which is chlorine AND ammonia bound together.
If you're going to risk a smaller tank with daily/bi-daily changes be sure to get additive materials like stress coats.
all aquaria should have trace salt in them always. We're talking 1tsp per 20gal range. All natural water has a small amount of salt in it.
Distilled water, Reverse Osmosis water are both no-no's. pH swings alone could have done this.
Six months is well within the range of minimum life span of mass bred betta.
There are two reasons we all insist on using larger volumes of water:
Betta can play and swim and have fun.
Things go wrong slower.
I add to this that a startled betta in too small a container is constantly bumping things, they need to swim freely.
This boy died of endocrine shutdown, likely from poisoning. My candidate is the tank itself, not the conditions in it. These are cheaply manufactured things made of hardened plastic, they contain chemicals that leech into the water in reaction to temperature, photo, chemical and pH stresses.
Acrylic tanks should be the only plastic tanks you ever use.
Yes, he was pine-coned.