Can you explain this goldfish behavior?

[janeslogin]

Wife has four Black Moors, 2-3 inches, in a 50 gallon tank.

One of the larger ones was laying on the bottom at one end of the tank looked as if it were dying. Water tested OK but the fish laid there all day so we moved into a 10 gallon isolation.

Twenty-four hours later the isolated fish is swimming around in a healthy manner and all three of the others are laying on the bottom at the same spot at one end of the big tank most of the time. They do sporadically move around normally.

What are your thoughts?