Certain species of bacteria can infect fish internally causing an array of symptoms. Usually manifesting as lethargy, clamped fins, reddening of the body and fins. If a fishes scales appear to be raised and their eyes are slightly bulging or protruding these are signs that your fish may have a different infection usually referred to as cloudy/pop eye or dropsy.
Fish affected by swimbladder issues struggle to stay buoyant. They typically look well but float near the water’s surface, or sink to the bottom, wobbling about on their side or upside down. They can get exhausted trying to stay upright, or trying to feed. This can be caused by various problems including a sudden change in water temperature although a swimbladder bacterial infection is a more likely cause.