You could try raising the temperature on your fish with the problem. I know they are cold water fish but the veiltails especially because of body shape do suffer from swim bladder problems. I do not generally treat any of my goldfish with any medications but I do have a bottle of Esha 2000 on standby in my fish-house just in case of problems such as this.

Check out the Bristol aquarists site, this is very informative and has some great pictures of British bred goldfish shown at some of the major coldwater shows around the country, I have pasted the url to the diseases section of the site, I hope it helps.

http://www.bristol-aquarists.org.uk/...o/diseases.htm