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.
Chat with our AI personalities
You can just search online for mermaid tails.That's what I did and I got an awesome mermaid tail.
make the sims toy want to be fell in love with drink it. by clicking ont them then click drink love potion x
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 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