For me it was running into velvet.
I wrote this as a reply to another post but thought it was worth its own thread. I have 20+ years in the hobby, and multiple tanks, one that has been running for over 15 years. Not once did I quarantine in any of my tanks. It was just unheard of back then. If fish died they died, there just wasn't as much information as we have available today. We would lose a fish here and there, but never anything major. I had the same mindset that most have, that quarantine did more harm than good. That was UNTIL my first encounter with marine velvet, luckily it was in my smaller tank with less fish.
Watching all 3/5 of my fish die in an established tank within a few days was an eye opener. I got them into copper as soon as I could and managed to save 2. If this had happened in my largest tank I would've lost thousands in fish as there was no way I could've treated all of them, I simply did not have a big enough system I could've put them into to treat for velvet.
Whenever I encounter a new problem I always do plenty of research and I learned quite a bit about fish disease at this time. I don't care how long your system has been up, what you feed, etc. If velvet gets introduced into the display it's a death sentence for all the fish unless theyre treated with copper. Many other infections, parasites, etc can be fought off with a healthy immune system, even ich, but not velvet. It is simply a matter of luck to not encounter it without qt and it's a chance I will never take again.
So the main argument, is that fish don't have a problem in the ocean so why can't they fend it off in the tank? To understand the difference you must understand how the parasites work. Ich is very similar to velvet with its life cycle and reproduction but velvet is far more deadly.
So first what happens in the ocean?
When a fish fish first gets infected by even a single spore, the effects would mainly go unnoticed aside from some flashing. The fish will not have trouble breathing and will still have an appetite. The spore will works its way through the life cycle until the tomont bursts and releases the spores, they become free swimming. Now in the ocean the odds of the newly born spores finding the same fish or another fish are so small that it will hardly ever be deady in the wild. Velvet and ich parasites do neglible harm in the ocean.
So what about in our tank? Let's say you add a fish with a single trophont that you missed visually, they can be harder to spot than ich in the same stage. That trophont will turn into the tomont and release spores. Now instead of those spores having trouble finding the next host in the wild, in the aquarium THEY ARE GAURENTEED TO FIND A HOST. Now they can infinitely reproduce at alarming rates, with extremely high infection rates killing fish very fast.
So here's my advice, if you can afford to lose all your fish, are able to move all of them into a medic tank or have a small tank with very few fish, it may be worth the gamble. In a large display tank with lots of fish I will never put them at risk betting on chance of luck.
So the next topic is losing fish in quarantine. Everyone says it does more harm then good, but Ive also learned it has alot to do with how the qt system is handled. If you treat it like another display tank I feel you will have better success. I cycle my qt tank and use live rock to set it up, monitor ammonia levels daily and use an ammonia alert badge. Ammonia is the biggest killer in a qt system. Second make sure you have plenty of aeration and water movement. And my biggest recommendation is to not immediately begin treating a fish. Let the new fish get comfortable and start eating before any treatment. I'll observe for any signs of stress or illness and if something pops up I'll treat for that first, if not I start copper. I bring up copper levels over a week instead of the recommended 2 days for most copper treatments so it's a little easier on the fish. After the copper treatment for 3 weeks, Ill add carbon or cuprisorb to the tank to bring down copper levels, do some water changes and then treat with prazioro. I haven't lost any fish in qt since I started it.
Hopefully this will help some of you make the decision on whether or not to qt.
I personally believe that, if a person is able to QT, then it is almost a moral responsibility to do so. These fish went through hell to get to my tank, and I am going to make sure I give them the best life they can have.