Sick Goldfish / Fantails

Out of the 8 fish we bought 2.5 years ago, only one fantail remains. He is 5
inches long including the tail.The last fish died about 3 weeks ago. Swim
bladder infection by all accounts, swimming on its side and upside down
though showed no distress before his death.

We bought 5 new small fish so that the last fish wouldn't be lonely. They
are also fantails.

The old fish swims vertically, ie head up tail down. He is very furry -
white fur all over him. We have put him into a separate tank but feel he is
on his last fins.

The 5 new fish are about an inch long and they are starting to become furry
Tail and dorsal fins), and though they do come up to the surface for food
they stay at the bottom of the tank most of the time and do not swim around
very much.

Can I do anything to save these fish, or are they all doomed?

Thanks.