This is my first year running salt and I had started with a 100g fowlr tank that was about 5 years established. The lady I got it from was not a fan of maintenance in any form, I was shocked her fish was alive. But even more shocked when her hippo tang died as soon as he was netted. Prolly in shock from seeing anything besides algae or pellets in his tank.
Anyhow I cleaned up the tank reset everything waited for the cycle to complete and added a yellow eyed tang and 3 chromis. Two days later down to one chromis... So of course I wanted more fish went to get chromis and got a fox face rabbit fish instead. He was over a year old and was huge and an awesome fish. Everything went great for about 2 months... Which was luck. I picked up a great looking coral beauty from fishy business and added him right to my dt. Within a week Every fish in the tank was over come with ich. Water params looked decent so I thought I had a chance to save em. Got called into work.... Came home after a 12 hour shift to a dead coral beauty. Not sure if it was just the ich, but he was covered. Regardless of what it was, he was sick. Which I would have seen with a proper qt time. Long story short even after pulling all the fish to a hospital tank my attempts at hypo salinity failed. Every fish died because I didnt invest the time into a proper quarantine, with a side of attempting hypo salinity. Which I will never do again, copper seems so much easier on the fish.
My tank ran fallow for close to 3 months, then after qting some chromis I started over. So my eagerness for the hobby cost me a lot of money in live stock, a lot of time, and I killed some of my favorite fish. Which damn near made me exit the hobby.
Live and learn, fish get decent qt stay, corals get a dip, and live rock is cured at home and ran in one of my extra tanks.
Ive read a lot of posts about ich or parasites or any other variety of things that go wrong adding anything to a dt. I ignored every single bit of it until it cost me a tank.
My set up now isnt perfect, but Im still learning the best ways to do things for myself. Which will always include livestock quarantine especially if from any of the LFS around here. From friends I will shorten the length a tad, but not much. Qt isnt just about watching for disease, but for helping them acclimate in a safe place where they can fatten a bit and get used to the difference in water/temp/lighting/screaming kids lol.