Originally Posted by Austin
I think another interesting thing you can do it be certain there's no disease is, if the tank is large enough, add one of your current fish. In case the newcomer is immune and a carrier you'll find out... It may sound mean but it's better than them all getting sick... That is, after quarantining for a while to be pretty certain it's fine. My thinking is like bringing small pox to the Americas. Native Americans who were not exposed to it before were especially susceptible. Think this could happen?
This is something that's often done in saltwater setups, many of which make my $500 look like chump change. When you've got thousands of dollars tied up in livestock you better believe many of them move fish from the show tank to the quarantine tank.
In my own experience, I had 6 chanchitos (cichlids) in a 55 gallon tank with a couple of schools. I added a school of giant danios that were quarantined for a month. In the next 2 days 3 of the cichlids died - fish that I'd had in that tank for 6 months. As it was I only wanted to keep 2 of them so it wasn't really a problem.
Sometimes things like that happen. Things you cannot account for, even though you do everything right.
Some people are able to dodge bullets for years and years. Good for them
Like driving without insurance, its all well and good till you get caught.
I will add though, that it's extremely rare that anything serious ever happens in quarantine in my experience.