After our goldfish had the meal of their lives yesterday morning, I noticed that Number 2 was looking a little peaky. He was gasping for breath at the top of the tank, lying on his side, dive-bombing to the bottom of the tank then slowly floating back up to the top again. Repeating ad infinitum.
I changed the water, much to the protest of Number 1, and pondered whether to put Number 2 out of his misery there and then. I didn't though - not sure why.
Today he is back to his normal self as if nothing had happened. Weird.