Sounds like you are having a bad time there safc
Unfortunately some strains of whitespot can attack the fish internally before any outward signs appear which makes treatment less likely to succeed.
Sounds like there is some fin rot going on too, probably because the fish have become weakened by the initial disease
I agree about doing a water change and gravel hoover and adding salt.
Waterlife do a specific treatment for goldfish that is for whitespot, velvet and fungus called Disease Right. But Protozin is also used and is very effect in most cases.
If you had tropical fish I'd suggest increasing the temp to 30, which does help, but I doubt your goldfish would thank you for it. I would guess that the temp is higher than it usually is though due to the warm weather. I know ours is.
I hope you can get on top of things and your fish recover safc