I would assume a couple of things here - 1, your LFS probably runs subtherapeutic levels of copper (meaning it would help to suppress issues like velvet at the store, but it wouldn't get rid of the issue), and, 2, your LFS probably doesn't run therapeutic levels of copper for a full 30 days for each fish.
So, I would assume that one of your fish came in diseased, and now that disease is in your tank - so, every time you add a new fish, it gets exposed to that disease while highly stressed from transport, and it dies too:
Personally, I'd pull your fish out into a QT tank (that's at least 10 feet away from your display tank), treat them with a chelated copper med like Coppersafe at 2.25-2.5 ppm for 30 days (it really just needs to stay above 2.0 ppm), then just have them stay in the QT tank until your display tank has been fallow for 60-76 days.
After the 60-76 day fallow period for the display and the 30 day copper treatment, your fish should be ready to return to the DT.
Since the copper treatment is only half as long as the fallow period, though, you might as well just run a full QT at that point:
2024 Quarantine Procedures Jay Hemdal David Scarborough Protozoans (Cryptocaryon/ich, Amyloodinium/velvet) and Metazoan trematodes/flukes are by far the most common parasites found on newly acquired fish. A carefully managed, proactive quarantine process can effectively eliminate these...
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