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.
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.
- This is the premise of Orenchi no Furo Jijou — a homeless merman named Wakasa takes up residence in Tatsumi's bathtub. The English title is literally Merman In My Bathtub.
- 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.
- Considerably larger than a mere bathtub, Meron's "bedroom" is literally just a full-size swimming pool.
- 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.
- 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?"
- InSplash, 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
- The Shape of Water, Elisa keeps the Amphibian Man in her bath, with some added table and mineral salts since he prefers brackish water.
- 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.
- 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.
- 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.
- 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 because Phoebe has a television interview, and they didn't want to risk exposing her as a mermaid.
- 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.
- 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)).
- 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.
- 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.
- 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.
- Mermando the merman in Gravity Falls 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 Sponge Bob 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.