I have 4 celestial eyed goldfish, one is 8 years old. Everything was fine but then two days ago they started sitting at the bottom of their tank (55 gallon) and they seem to be breathing very laboriously. He has 3 room mates, two much much smaller celestial eyed goldfish and one medium sized celestial eyed goldfish. This morning one of the smaller celestial eyed goldfish was dead. The other small fish and the medium fish were lazy but I did a semi-water change and they seem much much more active. However, Darwin is still not moving around! I have a significant amount of air, a large filtration system, I've done water changes... Help me please!!