Hello, I have 3 goldfish in a 40 gallon tank. One of the fish is 2.5 years old. For the past 2 days, he is just sitting on the bottom and doesn’t eat. He swims around alittle but just stays on the bottom and when he swims he looks like he is off balance and leaning to one side. He looks normal and all his fins look fine except the one on top of him looks pushed back. I checked all the levels and everything is fine. I also just completed a water change. Any help would be great.

Thanks,
John