Help! My beautiful yellow-eyed Kole tang was doing fine yesterday, behaving normally, eating beautifully, and then today it was lying flat on the bottom of the tank looking like this. It was gasping for breath, clearly in distress. In the time it took for me to move it out of the tank to a quarantine tank he died. All of the other fish in my tank are behaving normally, looking fine. I haven’t added any fish to the tank in months, though I did add a few snails a few days ago. I’m fighting a moderate cyano outbreak right now, as you can probably see in the picture, but other than that my water seems OK. I’ll post my parameters below. Any help with ID would be much appreciated, as well as recommendations for treatment to prevent whatever it is - I’m guessing velvet? - from spreading to the rest of my tank. Thank you!
temp - 78.4
ph - 8
salinity - 34.7
alk - 9.45
calc - 439
mg - 1300
ammonia/nitrite/nitrate/phos - 0