what went wrong

[gobytron]

seems like you should remove the coral.

just because you take the time to acid bath and then wash them, they can still have all kinds of stuff that can leach into a body of water.

Not sure I'm following this correctly, but it would seem like all your problems started when you added the coral.

The fact that it was from a garage sale is somewhat concerning.

There could be copper or other hazardous trace metals, chemicals or compounds in there and you would have no way of knowing until you have an issue (which seems to be the case).

Lots of people like to add coral to their freshwater tanks, and if this was the case, may have dosed with all kinds of stuff that would be very detrimental to a sw ecosystem.

they could also have been stored somewhere and exposed to something hazardous (like a garage or under the kitchen sink).

I'm if following this right, get rid of all that dead coral you added, put your live rock back in, add some bacteria (like prodibio) and do lots of water changes until things start to improve.