I have an (inactive) mermaid sim that just keeps reverting back to human. Is there any way to prevent this from happening?
The first time I made her into a mermaid was with kelp,when I noticed she had turned back human I changed her into a mermaid with MC.

I don't have that much experience with mermaids (much less inactive mermaid residents), but don't they turn back to human if they don't swim in salt water every 2 days? I don't know how the game handles inactive mermaid residents. Have you tried giving the sim the "Permanent Mermaid" LTR?

In one of my games I have a mermaid teen for the first time. I'm assuming that if I don't get him that LTR he'll probably end up a human sim once he ages to YA and moves out of the house. But I really don't know.