Good idea, Annette!
I come from the train of thought that parasites are ALWAYS present in fish shop tanks. If not always, then 90% of the time. With that in mind, I always treat incoming fish as though there are multiple parasites coming in with them. Luckily, there is a way to treat for 99% of parasites without harming your goldfish. Below is my procedure for safeguarding against future parasitic invasions:
--------At least a ten gallon tank with a filter that pushes 15X's per hour (ten gallon tank would need 150gph to reach this level).
--------Salinity is raised over the course of three days to 0.3% (zero point three percent). The salinity remains at 0.3% for the duration of the one month quarentine period.
--------Prazi treatment is also administered as per the instructions on the bottle. I even double dose it because it is non-toxic at any level.
--------If ergasilus, lernea or some other parasitic copepod is visible anywhere, Dimilin is administered. I double dose this stuff as well, its non-toxic.
All three of these drugs are safe to use with goldfish and are also safe for the bio-filter. so, there is nothing to worry about except keeping the water clean and careful observation.
If after being in quarentine for 1 month and I have not seen any unexplained flashing or scratching, I consider the fish to have a clean bill of health and it can be removed to its permenent tank.
If at some point in the quarentine I see any unexplainable bacterial disorders, I immediately feed medi-gold until it is under control.
After reading just about every single bit of info available to us on the net, I have come to the conclusion that prevention is the VERY best way to keep from having parasite problems. If anyone has ever battled a parasite that you cannot see with the naked eye, you know how utterly helpless you wind up feeling before you can get a definite diagnosis.
I think a good point to my reasoning would be the fact that Rick Hess, a well known dealer in quality goldfish AND co-author of the best book ever written on goldfish, Also suggest treating for new arrivals in the same fashion I have outlined above. That is, unless you happen to own a microscope and are very familiar with all of the possible parasites.......
Hope this helps!
Paul