Sick fish now this

[Debkai]

Hi all!
So this is in our quarantine tank—we had a sick fish with distended eye and belly who died. This is now coming out of filter. Ideas? And thank you in advance!!

 

[GaryE]

Biofilm. It's one of the stages in bacteria recolonizing a tank when the existent population was killed.

 

[Colin_T]

clean the filter or at least the filter case.