Small containers
Tiny containers are harder to heat and keep clean. A real aquarium is easier for both the keeper and the fish.
Skipping water tests
Clear water can still contain ammonia or nitrite. Testing is the only reliable way to know what is happening.
Overfeeding
Extra food quickly becomes waste. Feed small portions and remove uneaten food when possible.
Rushing tank mates
Compatibility depends on space, temperament, cover, and backup plans. Some bettas should live alone.