 If you liked
Chiisana Oningyo Biyori
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 ...then you might like
Oounabara to Wadanohara
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Wadanohara and the Great Blue Sea and Nice Day For a Little Mermaid are both 2-volume fantasy manga that features humanoid characters, including a few mermaids. They both have a cute chibi-esque art style and try to feel very wholesome as the main character tries to be generally kind to everyone. There is also particular focus on the main character and a male character who is a different species than her.