Posted: Yesterday at 19:46
Yes, I have seen this. Swim bladder problems can be serious. It is best to isolate the fish concerned in a separate tank or put it into one of those big plastic buckets that builders use. Fill the tank with pond water or fill it with water from a waterbutt. If that isn't possible, fill it with tap water and add Aquasafe, a chemical that makes tap water safe for fish. Then add something to provide safety - give the fish a bit of weed or a piece of old plastic down pipe to hide in. Cover the tank with a bit of garden netting to stop the fish from jumping out. Here is some help with how to treat the fish
Act quickly and the fish will recover, delay and it may die. The other fish are just reacting to the danger that the ill fish may present to them in terms of their own health.