I have had 2 goldfish for 2 years in my aquarium. This one was good last night. I don't know what happened today it is somehow floating on side with little movement, like sleeping. I put it outside of aquarium with filtered water now. When I bring my finger near it, it moves a bit, but very slowly, which is sad.
I didn't change the aquarium water for a while (maybe a year - my impression is they enjoy dirtier water, like pods - as you see it was full of their poop). But if it was for the water, the other one should have got sick as well.
What happened? Is she dying?
UPDATE 1 (1st week):
After a few hours of putting it separately in that bowl with fresh filtered water and a bit higher temperature, she got better now and swims. But the cause is still unknown to me. I changed the aquarium water and will get them back to it after a few days.
UPDATE 2 (2nd week):
After a week and a few days back in aquarium, I see the same thing happening. I got her back to a bowl now. I see her left side is always up when tilted (like this photo above) and when she swims, she gets upside down and has no balance. Is it swim bladder? I gave her pea pieces, but how much should I give her? And does it cure her?
Unfortunately she died, which is incredibly sad. I performed an autopsy to realize what was wrong. Here are two photos (viewer discretion advised). What was wrong?