I have had this before twice, I had a E yellow stuck in a shell and did not notice. I could smell it and then noticed the oil on the water. I just did lots of water changes and changed the filter wool every day for a few days.
Another time I had a male BN trapped in a shell from a female BN that was breeding with it, he was well dead and was sitting on eggs. She had another batch of eggs waiting for the dead body to do something with. (Stupid women...) Agin lots of water changes and changing of filter wool