very large acro dead in days

[Nyx]

We have a very, very large hairy Australian acro that we've had for a while in our well-lighted, well-established almost 10 year old reef. Nothing has changed in this tank but over the past couple of days but this acro went from a rich emerald green thriving coral to an almost dead, except for a few tips, coral. You can almost see it happening right before your eyes. What, pray tell, can kill such an acro literally in days?

[Hookup]

that is very sad to hear.  I've not experienced that myself, but seen many people post similar experiences.  I know that in ULNS ALK swings, and more specifically low ALK can wipe out a coral pretty fast, but usually it starts with bleached tips...

RedBugs and AEFW can do bad things too... but I'm assuming you'd have checked for those already...

[Nyx]

Definitely not parasites but possibly a dkh fluctuation. Man, I'm really, really fed up with this aquarium.