Sorry to hear about your loss. I don't know if I'm missing anything in this in terms of what you did prior to introducing the fish into your main tank. After my initial experience with Ich and fish dying when first entering the hobby, I learned after difficulties that a minimum month long quarantine in a completely separate QT tank is the best way to go. It saved me a couple of times when new purchases would come down with ich or velvet. With the ich, I was able to treat in the QT and make sure the fish was completely healthy before introducing it into the main tank. With velvet, the disease happened so quickly and two weeks after I first put the fish in QT that there was nothing I could do. It was definitely a pain to not have the new purchase in the tank immediately, but after losing so much livestock initially, it ultimately became worth it. No matter how good an LFS is, the fish is exposed to different occupants in the tank from different shipments and origin points over the course of the time that it is there. Even if it's been there for a few weeks, there's nothing that keeps ich or other diseases from the day's shipment to get onto the livestock already in the tank. Again, I don't know if multi week quarantine is already part of your normal practice, so apologies in advance if this is already the case..
