My Girlfriend is a Mermaid!? Refine

Tired of your normal city life? Maybe it's instinct, maybe it's science, but nature is calling Hiroto back to his rural hometown! Little does he know that his childhood friend has somehow gotten herself turned into a... mermaid!? She has fins...? She has fins! Never underestimate the power of science... Leaving the big city to return to his rural hometown, our protagonist reunites with his childhood friend Ion. But she's now... a freshwater mermaid!? It doesn't end there though, he soon meets a...