Need help identifying cause of fish death

[Hugh Mann]

Also could leaving the skimmer off for one day have caused this? I've run a tank with no skimmer for a while but it didn't have much livestock at the time... When I did the water change I turned off the skimmer with the intention of cleaning it and left it off for the day.

+1 to potential oxygen issues. Larger, more active livestock would die first. What else to do you have in the tank to facilitate gas exchange?

 

[Destinthecrab]

I have a Tidal 75 HOB filter that gives me quite a bit of surface agitation ...

 

[Destinthecrab]

+1 to potential oxygen issues. Larger, more active livestock would die first. What else to do you have in the tank to facilitate gas exchange?

I think it makes sense...of all the fish my lawnmower has more idle time than the other fish did. And maybe that's why it barely survived. Inverts I guess require less oxygen???

I don't think there is anything toxic in the water because my hermit is still alive and well in there...

Something triggered the algae bloom that overran my UV sterilizer, which reduced oxygen levels. It probably did not help that the skimmer was off as well...I'm guessing the extra nutrients fed the bloom.

Can a bloom occur after scrubbing thick algae off of the filter?