What happened?

Sounds like ich for sure. First off your tank is way to small for any Tangs, much less three. I assume you did not QT. Someone probably came in with ich just not visible. The stress of so many tangs in such a small environment lowered the resistance and your fish all died because of it. This would not effect your inverts. Please do research before buying fish. I'm no tang police member but this was an uneducated move that cost the lives of all your fish.

Maybe the worst part, to get rid of ich your tank must be fallow for 72 days, or hyposalinity which will kill just about anything left in your tank.
 
Way too many tangs in too small of a tank (perhaps one of the yellow tangs would have been marginally acceptable). Chromis are known to kill each other off in a smaller tank until only one is left. One pair of clowns is all you can keep in anything but a very large tank (180 gallons minimum, but better odds in an 8' tank). By your description it doesn't sound like you quarantined your fish, a sure fire way to wipe out a tank with ich or other diseases. I would strongly advise you to ask about appropriate fish selections before you purchase fish and please be sure to quarantine them before you add them to your tank. I would leave the tank without fish now for at least 72 days for the disease(s) to die off, then slowly reintroduce appropriate fish.
 
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