After reading this question regarding how to make a Cantonese fish soup I was considering adding an answer expounding on the fact that fish stock shouldn't be left simmering for too long, or you'll get a "glue flavor" in your soup because of bad tasting compounds being released from the fish trimmings.
However, upon googling for some sources to validate my claim I came upon this article, which among other things claims that
If no flat-fish bones are used, the stock can cook for four to six hours; this slow cooking extracts all the gelatin from the bones and makes a wonderful, rich broth.
Is it correct that it is only certain kinds of fish / fish trimmings which will create a bad tasting stock if left to simmer for too long? If so, are there any other kinds of fish than flat-fish which can create this bad taste?