Isekai Suizoku-kan no Tsukuri kata ~Monster Musume-tachi to Mujin-tou de Rakuen Souzou Slow Life!~

This is the ultimate narrative puzzle! We are now combining: The overworked aquarium worker, The Isekai/Fantasy world, The 1.5 billion yen inheritance, The "cost-effective" deity/monster girl, The ex-muse lookalike, and The revenge/tattoo plot.

Here is the combined English synopsis:

The Bottom-Shelf God & The Aquarium of Hell

Sotaro (formerly Muwan) led a life defined by sacrifice. After five years of grueling caregiving for his mother and working overtime at a failing aquarium, he was left with nothing but a broken heart and a shocking inheritance: a bank book with 1.5 billion yen and a map to his estranged father in a distant, shimmering "Pleasure City."

But the "City of Pleasure" isn't in his world. After collapsing from exhaustion, Sotaro wakes up in a fantasy realm where he is immediately exiled to a remote island with a supposedly "useless skill." Desperate to survive and reclaim his 1.5 billion yen, Sotaro performs a "Do-It-Yourself" summoning ritual. He expects a guardian; instead, he gets Wuyi—a "cost-effective" Legendary Sea Dragon Monster Girl. To his horror, she looks exactly like the ex-lover/muse who betrayed him in his past life.

Wuyi offers him a dark deal: she will help him build his "Ideal Aquarium" on this island—but it won't house ordinary fish. It will house the monsters of the deep. As she etches a mysterious lotus tattoo over his heart—a "secret" that binds their fates—she whispers a promise: "Don't just build a park. Use your billions to lure your enemies here and drown them in your own personal hell."

Now, the "Trash-Skill" exile and his "Budget" Monster Muse must navigate a world of greed and magic. Between the bizarre tension of Wuyi’s familiar face and the dangerous creatures they collect, Sotaro will realize that in this world, nothing is free—and the price of his dream aquarium is written in blood.