What do you do when your mermaids/Fish People/other aquatic sentient critters can't survive out of water, but you really want to take one home with you? Simple! You stick them in a fishbowl! Or a bathtub! Or anything else that can hold water, really. This allows the aquatic character to interact with the usually human (or at least terrestrial) cast without having to come up with an elaborate new set or location, and there's usually some humor to be mined from a fantastic creature chilling in a mundane fixture like a tub or tank.
This may or may not be a good idea, depending on the tone of the work. More precise logistics (such as the necessary salinity or chemicals present in the water) will usually be glossed over. While this can be played for comedy, more dramatic works' use of this trope will emphasize that the creatures are essentially captive in such a small space, especially if said creature was placed there unwillingly.
A sister trope to Mobile Fishbowl, distinguished by the fact that these containers are (usually, mostly) stationary. They may or may not be absolutely necessary to live, but they're at least important for the aquatic person in question's health.
- A variant in Banana no Nana. Due to how the world of the series works, characters can get various powers but have to go under certain conditions to wield them. The Starter Villain of the series is a human and the chief of the village the characters stay in who can wield water powers. But has to stay submerged in water and thus gets around in a wheeled bathtub. She can leave it, but only in short intervals and will die otherwise.
- This is the premise of Merman In My Tub — a homeless merman named Wakasa takes up residence in Tatsumi's bathtub.
- Monster Musume: Considerably larger than a mere bathtub, Mero's "bedroom" is literally just a full-size swimming pool. Later on her mother has it remodeled, adding an underwater basement that's bigger than the rest of the house combined.
- Dryden Fassa is introduced in The Vision of Escaflowne interacting with a captive mermaid in a large water tank. It then revealed that he purchased her in order to set her free.
- In Elric: Making of a Sorcerer, the prequel comic to Michael Moorcock's Elric books, Chaos Lord Artigkern has captured the sister of King Straasha. Both Straasha and his sister being water elementals, Artigkern holds her in a container that's described in the original script as "a gigantic semi-transparent clam shell". Also, when Elric and his companions, Dyvim Mar and Queen Shyrix'x, attempt to release Straasha's sister, they break the clam shell container and haul the water elemental in another spherical one.
- Hellboy. Abe Sapien has a large saltwater tank at the BPRD headquarters. He doesn't need to be wet to survive, but he does need to be submerged in good clean water every so often to stay healthy.
- Superboy: College-age Clark Kent meets yet another LL, Lori Lemaris, in college. She is in a wheelchair, but secretly she's a mermaid and spends her time at home in the bathtub. Clark finds out when he peeks at her using his X-ray vision.
- In Ironwood, Ignagio Pec has a mermaid living in his hot tub as part of his harem.
- Wonder Woman:
- Comic Cavalcade: Gerta von Gunther created a group of winged mermaids while experimenting on sharks and kept them in small tanks for observation. They resented her almost immediately as she'd given them human intelligence and eventually escaped and tried to murder her and attack the Amazons as it was an Amazon facility.
- Volume 1: After shooting Di and Steve with his Kal-C-M Ray Uvo dumps them both in a giant fishbowl with a couple of angry aquatic extraterrestrials.
- Referenced in an old Doonesbury strip. Zonker has gone to Ft. Lauderdale for spring break, and is seeking to share an already overcrowded hotel room. The room spokesman tells him the rules, which end with "Oh yeah, and the guy in the tub thinks he's a mermaid; you cool with that?"
- In the humanised My Little Pony: Friendship Is Magic fanfic Guppy Love, Applejack discovers an unconscious mermaid (Rarity) on the beach with a tail injury and carries her to an abandoned aquarium, where she places her in an old bathtub.
- Universe Falls: In "Monster Falls" Mabel is temporarily turned into a mermaid, and suffers the same problems as Mermando in being unable to breathe without water. The other Mystery Kids end up carting her around in a wading pool balanced atop a little red wagon.
- The Adventures of Sharkboy and Lavagirl has Max relate his story about meeting Sharkboy and keeping him submerged in his shower, which had been converted into a fish tank. However, Sharkboy can walk and breathe on land, and later in the movie it's shown he can't stay underwater forever or he'd drown.
- Inverted in Aquamarine, where the mermaid can easily turn into a human if her legs are dry (not so much in the book it's adapted from). But she does end up in the bathtub when the love interest comes calling...and her human friend is hiding under the water, passing off her legs as Aqua's.
- Deadtime Stories: Volume 1: In "Wet", when Swann visits Jack's home, he finds that Jack has been transformed into an inhuman merman by the mermaid, and is trapped in his bathtub.
- Miranda puts its titular mermaid heroine in a bathtub when she comes up to London.
- Mr. Peabody and the Mermaid has its half-fish lead end up in a bathtub.
- Pirates of the Caribbean:
- Davy Jones can only step on land for one day every ten years. As such, when having to hold a meeting on a sandbank in Pirates of the Caribbean: At World's End, he circumvents this by standing in a tub of water.
- In Pirates of the Caribbean: On Stranger Tides, Blackbeard and his crew initially carry Syrena the mermaid around in a makeshift tank. However, once it breaks, she develops legs.
- The Shape of Water, Elisa keeps the Amphibian Man in her bath, with some added table and mineral salts since he prefers brackish water.
- In Splash, Madison, a mermaid whose tail becomes legs & feet when she's dry is shown relaxing in her mermaid form the bathtub, the film even shows her adding table salt to the bath she's drawn.
- She Creature: A mermaid is caught and held in a tank on land. Later she's put in a tank on a ship and taken on a sea voyage.
- The children's book The Call of the Swamp by Davide Cali and Marco Soma depicts an anthropomorphic axolotl adopted by humans. While he can do all sorts of dry-land human activities without trouble, he sleeps in a bathtub at night instead of a bed.
- Our Bloody Pearl opens with siren Perle suffering after being held captive in a tub by a ruthless pirate captain.
- Vodyanoi from Perdido Street Station and other Bas-Lag novels need to keep their skins wet, so those which live out of water spend a lot of time in tubs. As it's a Crapsack World setting, the first example in the series is a slovenly old grouch whose tub is filthy and stagnant.
- Charmed: The second half of the season 5 premiere had Piper and Paige hide Phoebe (who turned into a mermaid) in their bathtub◊ to prevent her from running (swimming?) away again. They even use bubblebath foam to hide the mermaid parts when she has a television interview with Nancy O'Dell ("don't worry, she isn't showing any tail").
Phoebe: You are holding me hostage!Paige: Looks like it.Phoebe: I am not a common goldfish. I cannot ignore the call of the sea.Paige: Well right now, the call of the common household bathtub will have to do.
- Inverted in H₂O: Just Add Water. The mermaid characters are humans by default, and a special mermaid tail had to be made for all of Cleo's scenes that took place in the bathtub because she'd turn into a mermaid upon contact with any water.
- Sabrina the Teenage Witch:
- A Season 7 episode has her discovering a man who has magical creatures locked up in his basement. The mermaid is of course in a bathtub.
- The spin-off movie Sabrina Down Under sees Sabrina and Gwen finding an unconscious merman on the beach. They take him back to Salem's hotel room - as he has a full suite with a larger bath - and pour lots of salt in the bath to make him comfortable.
- Seriously Weird: In "Harris and the Mermaid", Harris meets a mermaid named Muriel, who promises to teach Harris to swim. Harris takes care of the mermaid, but Muriel has ulterior motives. By taking Harris' lock of hair, and with a magic kiss, Muriel transfers her tail to Harris, and Harris' legs to her. Muriel then leaves Harris into a leaking bath, which would eventually kill him, since the merfolk's tail needs to be kept wet. Fenella and Hugo find Harris in time and take him into a bathtub.
- In Vitas' music video for "Opera #2", he appears to be some kind of Apparently Human Merfolk who spends a lot of time in a bathtub.
- The Mili song of the same name somewhat deconstructs it with a metaphorical mermaid, who needs to lose parts of herself to make the human let her stay in the tub and becoming trapped once he loses interest in her.
- Leia from Harvest Moon DS and its Distaff Counterpart Harvest Moon DS Cute is a mermaid that washed upon shore. The local Mad Scientist Daryl takes care of her in a huge tank in his basement. If you marry (or befriend her in the Japanese version of Cute), she will move into a pond on your farm (and still somehow manages to give you a normal human child, which gets lampshaded by Daryl)).
- In The Sims 4 expansion pack Island Living, merfolk Sims show their tails when they take a bath.
- Sheba from Xenoblade Chronicles 2 is a Blade who seems themed around this trope, despite not being a typical mermaid. She constantly lounges about in a levitating bathtub, even incorporating it into her attacks.
- Horrifically deconstructed in Mermaid Swamp, in which the powerful Tsuchida Family have a legend that one of their ancestors met a mermaid while out hiking and, smitten, brought her home with him. Despite his best efforts at the aquarium he'd arranged for her, she died. It wasn't a mermaid. It was a human he saw swimming. He abducted her and kept her locked in an aquarium because he liked the look of her in water. She died from hypothermia, edema, or infection. He transferred her body to a septic tank to look at it for as long as it would last, then went to find another girl. Worse, his fixation was passed on to each male descendant and they've been abducting and murdering women for generations. Even the ones who have the moral reasoning to object don't have the willpower to fight the obsession and at best don't add to the collection themselves. Wives are kept in the dark and if they find out and object, they'll turn up dead somewhere sooner or later.
- In The Legend of Zelda: Phantom Hourglass, the Old Wayfarer is fascinated by a mermaid he's seen and wants to bring her home to live in a small pool he's prepared. Link finds the mermaid, who's excited by the prospect and seeks out the Old Wayfarer and can later be visited in the pool. She's not really a mermaid, just a human who likes to dress up as one.
- Parodied in Cyanide & Happiness when a mermaid dies after being put in a bathtub due to it not being saltwater.
- Pocket Princesses: Ariel the mermaid is often seen in bathtubs or large fishbowls so she can interact with the other characters.
- Monstra: One is seen in a bathtub when Kiwi goes looking through the house for the girl Storm was feeding on.
- Played with in Three Little Fish and a Bird, where the three mermen, Dace, Kiyi and Varden, live inside human writer Morgan's house. While they do prefer staying in water, they are obligate air-breathers based on dolphins who don't exactly need to be in water to survive. They are also quite able to crawl around the house and are quite mobile on dry land even without aid, thus doing away with the "confinement" part of the trope.
- The Amazing World of Gumball: Darwin walks on two legs and spends most of his time on land, but he sleeps in a fishbowl. If this is for his health or just comfort (it's the same fishbowl he lived in before growing legs) isn't specified.
- If it's for comfort it can't be much because a few episodes have made a point that he wishes it were bigger.
- American Dad!: Klaus the goldfish is usually seen floating in a fishbowl or lounging in a filled cup. In "1600 Candles", he becomes mobile by having a hamster ball filled with water. In one episode, Francine lampshades his lounging in a cup as unrealistic.
- Gravity Falls: Mermando the merman in "The Deep End" can't breathe without water, so Mabel ends up transporting him in a cooler.
- The Powerpuff Girls: Bubbles brings a baby whale home, so they flood the basement and hide her there from the Professor, who had warned Bubbles about bringing animals into the house.
- Captain Kirk and First Officer Spock from Star Trek: The Animated Series episode "The Ambergris Element" fall victim to a sur-snake while exploring the planet Argo, and are rescued by the natives, who convert them to water-breathers to make recuperating easier. Unfortunately, while this saves their lives and restores their health, Kirk and Spock must reside in a room-sized water tank aboard the Enterprise. This doesn't sit well with Captain Kirk:
Kirk: I can't command a ship from inside an aquarium.
- A live-action segment of SpongeBob SquarePants showed Patchy the Pirate talking to a mermaid guest at his house party sitting in a comically small inflatable pool, stating that if she stood up, she'd die. Patchy accidentally sticks his hook hand into the pool, deflating it.
- Punky Brewster becomes this when Glomer turns her into a mermaid to help her win the role of one in a school play (episode "Fish Story"). She hides out in the bathtub so her foster father Henry doesn't find out.