I also just found an African Ropefish I bought over a year ago living in the filter. He disappeared the night i got him and i always just assumed he was eaten by my tiger shovelnose cat (who also happened to "accidentally" swallow a 9" clown knife)
I recently had a young fire eel with my young tire track eel- called them Boudin and Dunlop- Boudin was my sweetie- unfortunately he was also too curious and explored my filter attachment, and got squished.
i had an 8" peacock eel living in my filter for a little over a week, i thought he'd jumped out somehow, i went to clean out the filter and was scared half to death when he jumped out of there