@JoshM5 Hello.
I found a couple of tips in this thread, see below. I think taking them to a mirror changing their scale colour is the least intrusive one; and that the Cure Potion followed by giving them Kelp is a reset similar to the one @Karritz is recommending using mods. Personally I would not recommend aging them down in CAS in an unmodded game; but have not played in IP myself.
Thread on AHQ, see posts #4, #24, #26
https://answers.ea.com/t5/The-Sims-3/Mermaid-scales-and-tails-not-showing-up/td-p/1950671
Tutorial for NRaas MC reset by Karritz
https://forums.thesims.com/en_US/discussion/945000/how-to-add-scales-to-mermaids-who-are-generated-without-any
This thread will probably be closed down soon, since the forum has got rules on necroposting, but there is a help thread for new members here:
https://forums.thesims.com/en_US/discussion/893122/thread-for-new-members-to-post-their-sims-3-game-issues#latest
And you can also ask on AHQ :smile:
Edit: Assuming you wanted help - but maybe you like them just the way they are :grimace: