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.
My child's betta has been floating on it's side and cannot go down
Abnormal swimming patterns are usually the first distinct signs that there is something wrong with your fish. You may notice that your betta seems drunk, swims in circles, or drifts with the current. There are also those that may sink to the bottom and are unable to raise itself, or may be incapable of swimming toward the bottom. A sick betta may also float and stay at the water’s surface. A fish suffering from severe health problems may swim upside down with its abdomen facing toward the water surface.
In some fish it is so severe that treatment won't help
My male betta fish is in a gallon tank, and I change the water once a week. I woke up and I thought he was dead because he is floating at the top of the tank and sometimes he is on his side. But if I move the tank a little bit he swims to the bottom of the tank then floats right back up to the top....
My betta is floating on his side and isn't moving around very much