Well, the livestock is all gone but the equipment remains.
The country is gone but the landscape remains. In a poem.
The same idea.
Pause a bit and think about it and start over again.
Sometimes ALL over again.
For a mainly fish tank, sometimes there is a possibility that a particularily virulent bacterial infection wiped out all (more likely if you never used UV effectively). In such a case, it is better to sterlize, yes, use bleach to kill ALL and start over again.
Or if you have invertebrates you can isolate them and support them with a part of the filteration medium.
Obviously, this is something not to be taken lightly. Has there even been any sign of bacterial infection at all?
It can also be that there is a oversight in inadequate gaseous exchange, simply not enough oxygen. What did you use for vigorous gaseous exchange?
Otherwise your parameters should not cause many fish to die quickly.