How about Zodiac or some items from the fish series?
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Also, placing some lotus lamps will give your room a pink light that would work with your Mermaid series. Blue would also work, if you want it to seem like its underwater.
I used a couple fish set pieces, island items (sea globe, whirlpool bath, wetsuits, etc), and a Pisces lamp (it adds a nice blue tint to the room, and plus I'm a Pisces so I thought it was fitting lol)