Something something injured mer getting rescued by carewhumper who wants them as an exhibition piece in their nice big tank and not at first understanding any of carewhumper's efforts (quarantine tank! stitches! everything is awful!) but then being quite content with the situation
Then carewhumper gets their hands on a second much smaller mer (subspecies?) and puts them in the tank as enrichment for their beloved predatory rescue who would absolutely eat them if carewhumper hadn't asked them not to :) Little mermaid's every instinct does not trust (if they even understand) :) Predatory mer is in their small little home range where they know every nook and cranny little mermaid could hide in :) There's nowhere for little mermaid to escape to because it's an enclosed tank :) Predatory mer is having fun :)