You still talking about the 2 goldfish in the small (5gal?) tank?
Chances are, you have ich, which is brought on by water parameter problems.
First thing - check your water parameters, do a large water change & good gravel vac.
Second - treat with salt. You need to raise the salt level slowly to 0.3% over 2 days. <Add 0.1% every 12-24 hrs.> This is the detailed instructions (with ich pics for you to look at.)
Finally, do consider upgrading your tank - it is difficult to keep goldfish in small setup, and you run into problems like this constantly.