Mermaid Mansion Story
人魚館物語 · にんぎょ かん ものがたり
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
MIyako's mother remarries and leaves on her honeymoon. She tells Miyako that she is moving in to Mermaid Mansion to live with her step-brothers Dai, Kyo & Akira. While on her way to her new home she hears the neighborhood gossip that the Mansion is haunted. Thus begins Miyako's supernatural adventures with her new step-brothers.
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections