What would kill a Naso Tank quickly?

I had a Naso Tang since Oct 1, 2001 that was a beauty, healthy and very active.Two days ago I was feeding the fish and it did not show up as usual. I found it barely breathing behind the rocks. No obvious signs of disease. Later I found it stuck to the Tunze Stream Turbelle so I netted it out and put it in the refugium but was barely alive so I took it out.

The yellow tang, Sailfin tang and Cole tang are OK. The only new think in the past week is that I dosed a lot of magnesium (64 Fl Oz) to raise my magnesium fro 1170 to 1270. I also fed the fish Nori Sushi.

Could it be that it aged and died?
Tank parameters are good and stable.
 
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