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Mermaids who have not made it into the water after a few days do revert back to humans. Since you can't keep an eye on resident mermaids and guide their activities all the time, the way to prevent this is once they already have the occult status, use MC to grant them each 5,000 (I think it's that low) LTR points and then Purchase Rewards to give them the Permanent Mermaid LTR.

@igazor Thank you very much! I couldn't find the option to purchase Lifetime Rewards for inactive sims earlier, but I have found it now, so I will make sure I give the Permanent Mermaid reward to all the mermaids once I convert them back (after I've given them enough Lifetime Happiness, of course!)

Another thing, I'm not sure it would work, but with story progression, if they had the lifetime wish of seaside saviour, they might be more likely to get in the ocean, but maybe not.

@SimKeats Yeah that's a good idea, I might consider doing that for some of them too, as well as giving them the trait 'loves to swim' - it might mean I'll see them in the ocean now and then!

That is the problem with inactive sims, they don't pay attention to their own needs, I have had mermaid sims cycle in and out, with a few that stay with out needing to use the Master Controller trick, I just leave Story Progression on to have it adjust the town occult types and other interesting things.