I've always liked the idea of merfolk having extra fins(by this I mean fins like the dorsal and pectoral ones and not the main tail fins). After seeing this article about how fish use their many types of fins it only interested me more. To me it seemed like they would help merfolk swim more efficiently, but after coming across this article on dorsal fins, I began to question how useful they would be to merfolk. In the article they mentioned that their hands would serve the same purpose as the fins would while in the water, but both the question and answers did not give much information to go by. The merfolk in my world will mostly be the typical half-fish-half human type as of now but some different types will have variation in their anatomy. If some types of fins could be useful, then what types would be useful and which ones would not? It would be amazing for the merfolk in my world could have these fins, but I don't want them to do more harm than good.
I hope this question makes sense. Please let me know if I left anything out that should be considered. Thanks!