Hey guys, not my tank but I'm trying to help out.
This same thing keeps happening over and over again, despite large (90%) water changes. Animates tested the water and they said it was perfect. The tank gets no direct sunlight and has a single tube over it. Contains a couple of Angels, some Cories and a couple of Kuhli Loaches. No live plants, just fake ones, also driftwood and silica sand. Has only started happening after the filter's been replaced by the new HOB in the photo.
Thanks!