No there isn't, there are no mermaids in the Sims games.
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In the Sims 3 Island Paradise your sims can become a mermaid. In order to become a mermaid, a mermaid must give you the mermaid kelp or in life time rewards you can purchase it. Your sim must be human to become a mermaid (not a vampire, wolf, etc.)
You can't, there is no such thing as a mermaid potion. If you want to become a mermaid on the Sims the only way to do so is by downloading a pre-made mermaid Sim.
You can't actually become a mermaid but you can download mermaid outfits, that is the closest you can get to being one.
xm sims is where you find it
They can only have mermaid costumes. They don't look that real you can see their feet. If you want your Sim to be a mermaid, buy the Sims 2 of download a mermaid Sim from the Internet.