About 5 months ago I upgraded my 125G system to a 550G system. Re-QT'ed all my fish, corals, and inverts before introducing them into the new system. Lost a Two Spot Bristletooth Tang in the QT process but over 2 dozen other fish made it through QT without any problems. One of them was also an Achilles Tang. This guy was a champ, and everything was great. Then in the last couple months the water clarity went completely into the dumps. It's been running with UV sterilizer ever since the tank got cycled and it was doing pretty good for a couple months. Then out of no where the water just got cloudy and I'm not sure what went wrong. So last week, I got a Ozotech ozone generator because the UV and the carbon media just wasn't cutting it. I've also heard that a big system like mine should give ozone a try. So I hooked this thing up Monday night and 3 days later tonight, my Achilles Tang is dead. No cuts or wounds, no signs of parasites or disease, was eating like a champ just this morning then tonight I found him dead stuck side ways on the overflow. I think he might've died a few minutes before I got home because everything on him was fine, not even missing an eye. Fresh, fat, clean, and dead. I really don't know what else it could've been but the new ozone generator I just added. All my other fishes are fine and I've got over a dozen other tangs doing great. I have my ozone generator plugged into my Apex controller with an ORP range of 300 - 350. Maybe my probe is off and the actual ORP is higher than that? But all my other tangs are doing fine. Maybe Achilles is more sensitive on Ozone?