AU where Nixie was actually Rita's daughter. There's a couple different versions of this. One has Nixie being Harry and Rita's daughter, when Harry died Rita gave Nixie to Veridia to raise in Pod as a Mermaid without any knowledge of her true parentage.
The other version is that Nixie is just Rita's daughter, born and raised within the Pod the way all Mermaids are.
Scene: Rita giving Nixie to Veridia as a baby to take and raise in the Pod
"I want you to take her. It's not fair to keep her on land."
"Hybrids aren't always accepted."
"No one has to know. Please."
"What's her name?"
"Nixie."
A Nixie POV from when she's older within the Pod
"You're such a land person Nixie."
How could a Mermaid be a land person? Sure she liked watching them and hanging near land was quite interesting but no one knew she did that. And anyways land people had legs. She had a tail.
When Rita and Nixie meet for the first time in S1
The first time she saw Nixie she knew who she was. The same features were across her own face. There were differences of course, no Mermaid was ever entirely identical - apart in the rare occasion of twins - but as far as mother and daughter went, they were close enough it was unmistakable.
In one episode I noticed at some point Nixie had a plaster on her finger. So basically it's a cute, fluffy fic of Nixie getting a cut and Rita helping her put a plaster on it