We've had two goldfish for 5 years and both were in the picture of health until late last week.
One of them is still absolutely fine but the other (Nemo) has deteriorated quite rapidly. His fins and scales seem fine and undamaged but he is now lay on his side at the bottom of the tank just thrashing about intermittently.
A bit of internet reading suggests it may be a swim bladder infection and we've tried adding a treatment over the past 24 hrs but there is no change. Water levels checked and all appears fine. Tank is properly oxygenated and water changed 30% on a weekly basis. Weve tried adding salt and give him crunched peas on advice of the local shop but I fear we're losing him. The kids are quite distressed when they see him.