@Jay Hemdal is r2r resident expert on fish diesese,I know vetteguy very knowledgeable also but he already been tagged.
I'm no expert but If was me I would be thinking if you didn't qt in first place on your fish and your fish have built up an immunity to certain parasites thats great but a huge stress event like this could weaken your fish and then parasites/ dieseses got the upper hand to strike so if is the case then could medicate your fish whilst they in qt.i will post Jay's current diesese protocol 2021 in a moment if you want a read while you waiting for water to get made but please wait till jay replys to give an expert opinion.
And even if your fish not shown any signs of diesese or parasites in last years,they could still be in your tank but in low numbers or whatever .
Anyway good luck you got a big job ahead of you.
Maybe ask family or friends to help you to speed things up ^_^
Edit: current diesese protocol
2021
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...
www.reef2reef.com
I also see on first page you say lost 2 tangs and a angel and clownfish,tangs have thin slime coats so I read alot people say this is why tangs are known as ich magnets as thinner slime coats so more susceptible to parasites ( not saying you got ich but saying still get advice from jay on his opinion but this tank and its inhabitants had a possible huge stress event)
Also on this screenshot of your orange shoulder tang I believe it is,what's on it where outlined ?