The fish float to the top or struggle not to float to the top. Oftentimes, they are standing on their head and float towards the top again and again. This is not due to a disease - it comes from the fish swallowing air. Fish (especially goldfish) often eat food floating at the water surface very hastily, thereby swallowing larger amounts of air, causing the buoyancy of the fish to increase. There is no need for immediate remedial action. The air will escape of its own after a while. In the future, though, the fish should not be given any more floating food. It is better to feed sinking food. Food tablets that stick to the pane or sink are also good alternatives to floating food.