Swim bladder disorder is the most common reason for a betta fish to float on its side. There are numerous causes of swim bladder disorder, including disease, physical trauma, parasites, environmental factors and constipation.
The swim bladder is an organ that allows a betta fish to control its buoyancy. When the fish is experiencing complications with the swim bladder, it is unable to control the direction it floats or swims.
The visual symptoms of a swim bladder disorder are a swollen belly, abnormal swimming patterns, sinking to the bottom of the tank and sometimes a curved spine. According to About.com, feeding skinned green peas to a betta fish can help relieve symptoms caused by constipation.