Wow and sorry to read this reported. Plz send Charles an email and ask him what he thinks but it looks like the dose was higher simply based on the amount of clouding captured in the picture. Every time I've dosed flu the tank has clouded but cleared overnight and the effect on water clarity was more a haze and not the heavy clouding I see here.
Did you have an air stone in the tank?
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.