You might want to buy a O2 testing kit. The filter bacteria gunk you have described can form in response to neglecting to lightly dunk the filter media in tank water on a regular basis to rid it of rotting waste. I've had this 'gunk' grow in a Fluval 404 filter and it had reduced the O2 levels to 1 ppm (it was using the oxygen to grow and filling the tank up with CO2). At the time that this happened, I was cycling the aquarium with pure ammonia, and by the 2nd month of not giving the filter a light clean, this super bacteria (or gunk) had formed.
I suspect the above may have happened in your tank too.
How often do you clean your filter?
I suspect the above may have happened in your tank too.
How often do you clean your filter?
