[AI Man]

I quarantine for 2-3 weeks before I put my fish into our retail tanks for sale. They are tested and tracked everyday. I re-assess treatment daily. Large water changes are a big part of my treatment regime, although costly 20-30% daily water changes have a significant effect on the fishes health. As I also use bacterial disturbing medications it helps to eradicate Ammonia, Nitrite, and Nitrate spikes.
I also use Hyposalinity and various combinations of Copper sulfate, Methylene blue, Malachite green, and Formaldehyde. It depends on what I'm dealing with. I also use freshwater dips, and medicated dips as well. I'll admit I occasionally see diseases that are unresponsive to my efforts.

Seeing various diseases that can come from wild caught fish. I think it is very risky and surprising that more people don't quarantine.
It seems that sometimes stress induced disease outbreaks take 3-5 days to appear. In most stores the fish is sold and in your tank by then, not to mention with the additional stress of recapture and acclimatizing to your aquarium.

I will also say that many disease outbreaks are certainly hobbyist induced by poor handling, poor acclimatization, bad water quality, and non compatible tankmates.
I can't count the amount of times I've had a fish in quarantine for 2 weeks, then in our retail systems for weeks to even months and a customer takes it home and it dies within a few weeks(sometimes days). I can confidently say that the hobbyist is responsible for the unfortunate demise of the fish. When I ask them about their tank their answer is almost always that everything is "fine". I wish I could say it but my response in my head is usually "well obviously not". Now sometimes we are able to source the problem with water tests and discussing their tank and husbandry techniques.

Nobody is perfect and sometimes fish go into shock, or have weak immune systems, but I think this would only account for a very small percentage. The fish had to be pretty tough to get to the retailers tanks in the first place.

I guess my point is that I think a 10 or 20 gallon quarantine aquarium should be in every serious aquarists fish room. Sure most times you can get away without it, but when you finally get hit with disease, you would gladly turn back time if you could. People say but I can't afford not to. My response is look at the price of medication and lost fish. Plus as Quagmire stated a quarantine tank gives your new fish a chance to settle and fatten up before having to compete with more established tank mates.

I also notice people don't tend to make a big deal of their aquarium failures. They talk about all the fish they have kept and omit from memory the ones that didn't make it. Or they say "But he had (insert excuse)".

My point is that I think everyone should quarantine regardless if the store they buy from does or not. At very least a fresh water dip or medicated dip prior to addition to the main aquarium.