Pictus catfish sudden death?

[IHaveAquariums]

So I have a 29g tank w a pictus catfish and 3 serpae tetra, it was temporary until I can move them to the bigger tank. I had done a water change 4 days ago, and did one last night when I got my ordered canister filter. I set it up, took the HOB off as it was pretty loud. The canister had established media and was filtering ok, it's a 115gph Cascade 500. I had the spray bar on, and did a change because I had issues priming and it was pulling water out. Well fast forward to this morning, and my pictus is dead! Also turns out the filter stopped working over night. No sign of any disease, no discoloration, nothing. Not even his barbells or whiskers were curled. But he's laying on his side dead. Well now my serpaes are not happy and struggling for air it seems and aren't doing well, their color is turning dark. So I did some reading and realized I left the spray bar under the water therefore hindering any oxygen/co2 exchange. Could this really have killed him? If not any ideas? I'm afraid I'll have to put down one of the serpae because it's not doing well at all

 

[TMaier]

Lack of oxygen would kill the fish.

 

[V227]

It seems odd to me that the lack of circulation in a sparsely stocked tank, for a relative short amount of time, would result in the suffocation of the fish. That is a possibility however.
Perhaps the BB of the aged filter media was damaged or destroyed during the transfer process between the old HOB filter to the new canister. If you don't have a tester kit to check the water parameters, I would treat the situation as if you were starting at ground zero, start the cycling process again. Good luck.