Goldfishes are sick - please help recommend medicine

Hi,

I have an established tank - about 100 litres -5 fantails and the goldfish
have been living there for about 3 years happily. No diseases.

One of my relatives in her wisdom bought 2 other fantails and dropped them
into my tank without informing me first. They developed spots on their tails
and bodies - like white sickly bumps. I tried to quarrantine them and they
died in quarrantine - a secondary borrorwed fish tank.

The other fantails also seemed to get sick too. I put some generic aquarium
medicine - and they recovered - a month ago.

However the spots have appeared again - and this time some of the goldfish
tails have turned like angel hair -strands.

I went away for the weekend - and the tails seem to have recovered but they
still have smallish spots. Otherwise they look active.

Now what shall I do? What have they got? And what sort of food or medicine
can I give them to help them recover?