In addition to parasites there are many other pathogens you could be transferring in that appear in healthy looking fish. See CB1 recent threads on this at :

http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=20;t=003496;p=1

http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=20;t=003524;p=1

http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=20;t=003586

See CB1's first link wrt VHS (Viral Hemmorhagic Septicemia)which has been found in some fish and areas of the Great Lakes and note the order covers SMB. That alone would concern me without more info. LMBV is one that can easily be transported by healthy looking fish. I don't know (not sure anyone knows) how long you would need to quarantine fish for all of these.