There is never a guarantee of any fish being disease free, even those we ourselves quarantine. In fact, I would suggest that many hobbyiests who quarantine fish with the best of intentions still manage to introduce parasites into their display because the fish in quarantine does not exhibit signs of the infestation and therefore is not treated in quarantine (which is why some suggest you treat every fish with copper and prazipro before placing the fish in the display whether parasites are observed or not). I guess what I am trying to say is that ultimately the goal should be to best balance risk. A fish purchased from Drivers' Den has a very low probability of having a parasite (never heard of a single instance but sure it has happened). Therefore, in this circumstance, the risk associated with quarantining such a fish outweigh the benefits, particularly if you are dealing with a delicate species or have crampt or sparce quarantine accomodations which will stress the fish or make it difficult to maintain prestine quarantine conditions. Conversely, I have read that all other fish purchased in the trade generally have a 33% probability of having a parasite which certainly suggests that the benefits of quarantine substantially outweigh the risk of not doing so and introducing a parasite into the display. Also, as I did state above, quarantining a Drivers' Den fish may be appropriate when your purpose in doing so is not to avoid introduction of parasites into the display, but it is rather to get a highly stressed fish to eat and acclimate to captivity in a controlled environment free from any aggression from other fish already established in the display.
