How to turn existing sim to mermaid?

You don't get kelp from merfolk, that was in Sims 3. I sent my sim into the cave, and she came out with 10 kelp in her personal inventory. I'm not sure what I'm going to do with the extra.

You can get it as a reward for your satisfaction points. It only costs 500. Unfortunately, it's considered a food, so it can't be saved in a room to just replicate a bunch of times like I tried. ^_^;;