Swim bladder: The swim bladder is a gas-filled sac located in the body of the fish. It helps the fish to control its buoyancy, which is the ability to float or sink in water. When the fish wants to stay still, it can adjust the amount of gas in its swim bladder so that it is neutrally buoyant. This means that the fish will neither float to the surface nor sink to the bottom.
Fins: Fish also use their fins to help them stay still in the water. The pectoral fins, located on the sides of the fish, can be used to create drag and help the fish to slow down or stop. The pelvic fins, located on the underside of the fish, can be used to help the fish to balance and stay in position.
Body shape: The shape of the fish's body can also help it to stay still in the water. Fish that have a streamlined body shape, such as tuna and salmon, are able to move through the water with less resistance than fish that have a more rounded body shape, such as goldfish and carp. This means that streamlined fish can use less energy to stay in the same position in the water.
By using their swim bladder, fins, and body shape, fish are able to stay still in the water and conserve energy. This allows them to ambush prey, avoid predators, and rest without having to constantly swim.