A common problem we see, especially in betta fish, is known as Fish Bowl Syndrome. Essentially, it can be broken down into one main issue:
Your fish is in a bowl.
Problem #1: Your fish has no filtration for processing of dangerous ammonia waste. Without a place for bacteria to grow and filter the water, your nitrogen cycle can never be established. Some may argue that betta fish get pushed around by filters, but if you chose the right one and slow the flow, it is perfectly suited to bettas.
Problem #2: You fish has no oxygen circulation. Fish tank filters are great at adding in oxygen, even in slow-flow mode. In a stagnant bowl, your fish is limited to what air diffuses from the surface. Yes, betta fish have a specialized labyrinth organ used to squeeze oxygen from the air. This is a short-term adaptation mechanism used to survive in drought conditions. It is NOT a long-term survival technique.
Problem #3: Your fish has no temperature stability. We’ll pick on betta fish again. Bettas are tropical and need heat. The small volume of a bowl causes temperatures to fluctuate dramatically. Watch the difference in ambient room temperatures on a 1-gallon fish bowl and a 10-gallon fish tank. It’s the same as you stepping out into freezing temperatures wearing a t-shirt or a coat.
Problem #4: When doing bowl water changes, most often, you will take your fish out of the bowl and put them in a tiny container while you empty all the water out of the bowl and scrub it clean. Since you have no filtration or oxygenation, you can swing the water chemistry parameters, including pH and temperature, very rapidly, causing your fish to stress and possibly die.
Fish bowls are ancient technology invented before there was electricity. With all the modern updates to fish-keeping, isn’t it time to let the fish bowls go? Unless, perhaps, you use them for this purpose: