Sea Monster, Imprisoned By Madness. Boohouse Books Dark Stories, #4

Ormy lives a life of isolation and insomnia. One day as he walks the beach with his metal detector, he finds a miniature mermiad caught in cruelly discarded trash. He brings the treasure home, nurses her back to health and gives her a home in a fishbowl beside his bed. That night, he finds that the tiny siren's musical lulls finally get him to sleep after years of restlessness. Days later, an aquatic monstrosity rises from the ocean and slowly makes its way to Ormy's New York, apartment building.
Watching the events unfold from his luxury loft, Ormy wonders if the creature is after his new companion.