My Yellow Tang was gone. It died mysteriously

[TOMMY323]

My tank was setup almost 2 years. Everything was fine untill yesterday, I looked at my tank on 2'clock (after midnight and light goes off). I saw it on the sand like a leaf, and it does not swimming. I think at that time, it does not completely died. Because After 30-minutes, I saw it at another place but very near, like it dance. It don't know what happen to it. No disease (no white spot), no water change. Just an ordinary day. I did not do anything to my tank in 2 or 3 days except top off water, but i think the top off water may not a cause.

The following is my detail.
Tank size: 28x18x20
Fishes: 1xBlue Damsel, 1xTomato clown fishes, 1xYellow Tang (which was died) They stayed together for a years.
Temp. 82-84 everyday (no chiller, no heater)
ph. 8.2
NO2 and ammonia = 0
NO3 around = 5~8ppm
PO4 less than 0.1ppm

I leave in Bangkok, Thailand. My friends and LFS suggest that It would be shocked by something else. Is it true?

Anybody have any suggestion?

 

[EmilyB]

I just saw this...

Were you able to figure anything out?

How is your tank oxygenated. ? Low oxygen does NOT necessarily affect all fish, but will pick off the one who requires the most first.