Stagnant water (oxygen boiled out) OR a defective suicide rod (anode)
Or poor quality water entering the system and the bacteria is cooked inside this tank and left to rot and not being flushed out properly.
High sulfur content
Too low of a water temperature not killing off the bad bacteria and allowing all kinds of stuff to breed in this incubator.
Mind a suggestion? Get a water sample and have it tested.
Then you know what your facing and how to have it treated. Ta Dum