I was thinking about quarantining the other day, wondering how effective it really is. Now, I put all new fish through a heat treatment while in quarantine, since a fish can be an ich carrier without the telltale spots. Also, heat speeds things up, so if there are bacterial or other problems, they are manifested faster. I have recently decided to treat all new fish for internal parasites as well. Since one of the main functions of quarantining is to cleanse the fish, and since you can't always tell if a fish is carrying something by visual inspection, isn't preemptively treating the fish the only way to make sure you don't introduce something to the tank? Surely when animals are introduced into closed systems, such as a zoo, they are actively medicated and vaccinated before being released into the general population.