I like to have a bunch of non-human sims like vampires, aliens, genies, fairies, etc. in my sim worlds but I've always had trouble with mermaids since they go back to being human after a couple of days which ends up being annoying when I go back to the household. I have NRAAS and I know you can add the lifetime reward to make them permanent mermaids but is there a mod or anything out there that I can do something like this on a grander scale or keep them mermaids so I won't have to add the LTR to each individual sim and any kids they may have?

I'm not too familiar with mermaids since I don't have Island Paradise installed but have you tried changing the kTimeUntilDivingDeficiency value via Nraas Retuner? If you set it to a ridiculously high number, like 10,000,000 sim hours, it should effectively get rid of Salt Water Deficiency I think

How would I go about doing that? I didn't see any options when I opened the retuner via city hall nor computer.

Go to "Settings" (after you open Retuner via compurer/city hall), then navigate to:

General > By Tunable XML > Sims3.Gameplay.ActorSystems > OccultMermaid
There should be an option called "kTimeUntilDivingDeficiency" that you can change to your liking, the default value will be 48.

Honestly, Retuner is pretty tricky to get into but you can do a lot of cool stuff with it once you figure it out!

Thank you so much. I was looking in the wrong settings. It worked like a charm. Now I can go back to my mermaid households and not have to worry about them turning human on me.