That's part of why I'm so baffled.
The dose being too high would explain the cloudiness (but considering there's zero haze in my main tank, this isn't the magnitude of effect one would see from a maybe-10% OD) or a lack of O2 suffocating the fish would explain the deaths (since there was no air stone, just the HOB), but neither would fit the explosive-growth of the white stringy material.
There's enough of it covering the bottom of the tank that I suspect it's the main cause of the cloudiness, but I can't figure out what it could be that it would take over the tank that quickly.
WOW! I'm surprised as well and everything doesn't fit together.
The drug does scavenge Oxygen.
How about aeration and tank temperature? Do you keep the water surface moving?