There's a very useful tutorial series on youtube about rigging the mermaid tail, beginning with this: https://www.youtube.com/watch?v=pGloKeDgH4k In short: you will want to rig the tail to only one leg. So you have to clear the weightings from the other leg and give it to the first one. When you decide to create a well-fed mermaid, you will notice that the midline-vertices behave strangely. You can easily bring them back to their y-position in edit mode. No need to get frustrated ;) Use the diffuse texture for color and transparency. Use the normal map as usual. Use the spec map for spec color and spec intensity.
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