Karin Wilzig has a hard time choosing a favourite colour from among the 64 that she and her husband can use to illuminate the 14 1/2- foot, 450-gallon aquarium in their TriBeCa town house. The default is fuchsia, which turns the dozen koi a deep pink.

“Not pink,” said Mrs. Wilzig, 40, an artist and a mother of two small children. “Alan, go to the turquoise.”

Her husband, Alan Wilzig, 45, a former banker who collects motorcycles and prides himself on the orange tanning bed in his basement, goes to the James Bond-like control panel in the kitchen, where a touch of a button turns the fish — which are specially bred to be colourless — a vivid blue.

“I think they like that,” he said, walking down the steps to the sunken living room to admire the fish from another angle. (Given that they do nothing but swim from one side of the tank to the other, it’s hard to tell.)

Most people who keep fish have a tank or two; perhaps they start with a five-gallon model and graduate to the 35- or 50-gallon version that doctors put in waiting rooms to keep patients calm.

Custom aquariums are popular for two reasons, interior designers say. One is that upscale nightclubs, restaurants and boutique hotels have been installing them, which gives homeowners the me-too idea. Another is that, among people of means, a dazzling aquarium is one of the last surefire ways to impress their peers.

Christopher Stevens, a Manhattan interior designer, said he has worked several giant fish tanks into residential projects at the request of clients. “They have a collection of cars, of motorcycles, of art, they have three dogs,” Mr. Stevens said. “It’s like, ‘What else, what’s the next thing to wow my friends?’ It doesn’t seem like the kind of thing you’d see in high-end interior design, but that’s being reconsidered.”

He sees it as a way to add movement and fluidity to what might otherwise be an arid space. “One of my challenges in doing more modern residences these days is just sort of to soften things,” he said. “How do you humanize this space, how do you introduce natural elements? How do you make it feel like you’re not standing in a white, pristine, soul-less box?”

But all that movement and fluidity comes at a price. Universally, owners of fantasy fish tanks describe them — usually in the same breath — as very relaxing and very expensive. Aquariums like the Wilzigs’ tend to cost a minimum of $50,000, plus at least $1,000 a month for maintenance. And that’s before buying a single fish.

In the world of fantasy fish tanks, it is not uncommon to pay $600 for a black tang or $5,000 for a pet shark, or to have service people on call 24/7 in case a fish gets sick or dies, which could contaminate the entire tank.

“I get calls at quarter to 12 on New Year’s Eve,” said Ralph Ammirati, owner of Aquarium Network in Bellmore, N.Y. “We answer the phone.”

Joseph Caparatta, owner of Manhattan Aquariums, which sells tanks small and large from a showroom on West 37th Street, said that, increasingly, “Most of the jobs we get come from architects and designers who have to fill a 6,000- or 7,000-square-foot apartment.”

It was Caparatta who suspended a 700-gallon aquarium from the ceiling of a town house apartment in the West Village owned by Richard Wise and Andre Jones. The filled tank weighs at least 6,000 pounds and has cost the couple some $200,000 in equipment and service.

“At night, we sit in the living room and sort of get lost in it, instead of the television set,” said Jones, 40, who owns a construction company, Wise Builders LLC, with Wise. “It’s always the centrepiece of the party

The couple keeps bags of brine shrimp and sardinelike fish called silversides in their freezer drawer, next to the Häagen-Dazs and Lean Cuisines. “We feed the fish once a day,” Jones said. The equipment needed to support the huge aquarium — pumps, pipes, chillers — occupies a walk-in closet as well as part of a roof deck.

Their three-year-old tank has a salt-water coral reef filled with catfish, tangs, pink damsels and a two-foot eel that rarely shows itself. “Don’t ask me the names of the fish,” he said. “Joe gives them to me, and then I make up my own.”

He added: “At first, when we lost fish, we’d be all traumatized. Now we’re not quite as traumatized.”

Their apartment is on the market for $16.9 million, and some potential buyers have expressed interest in keeping the aquarium, while others have said they would want to remove it. Meanwhile, Wise and Jones have bought a new place nearby and are considering jellyfish for the dining room.

Jellyfish tanks are even more expensive and difficult to build than fish tanks, said Justin Muir, owner of City Aquarium, a Brooklyn-based rival to Manhattan Aquariums. For one thing, jellyfish have to be fed live food every day.

But “some people are like, ‘O.K., $5,000 every month to take care of the tank, plus $100,000 cost of the tank — I’m cool with that,’ he said.

Muir, a marine biologist who has been building custom tanks for 10 years, considers himself an artist who works with water, light and high-end cabinetry. His starting price: $50,000.

Forget miniature mermaids and artificial plants; he uses handmade sculptures and antique statues, as well as exotic fish, sea horses and coral. He even shops at ABC Carpet & Home, he said, “for crystals and all kinds of things we can sink into aquariums. We do a gem tank with all these minerals and geodes that are a perfect habitat for fish.”

Muir’s clients include C. C. Sabathia and Jorge Posada of the New York Yankees, plus plenty of hedge-fund millionaires. He also did a six-foot-long tank for the bedroom of Anna Anisimova, the daughter of a Russian metals billionaire, who lives in the Time Warner building; according to Stevens, who designed the apartment, the heiress was inspired by aquariums in nightclubs and lounges that she frequents.

The most expensive tank Muir ever built, though, was a $750,000 one for a woman in Dallas who had visited the Maldives and wanted to recreate the experience of lying in tropical waters gazing up at the stars. She had a planetarium ceiling and crescent-shaped aquarium panels hoisted by forklift into her second-floor bathroom.

But the real expense of owning such a fantasy tank is the maintenance. Muir has a staff of seven technicians and biologists who make house calls. “Some clients want nothing to do with the fish tank — they don’t want to feed it, they don’t want to clean algae off the glass,” he said. And most fish should be fed at least every other day. “That’s $150 per visit right there.”

Among Muir’s favourite tanks is the one he built in 2005 for the Wilzigs in TriBeCa, for which he charged $37,000. (“It should have been more,” he said. “But I was just starting out.”) His goal was to play up the home’s tricked-out lighting system by making the tank translucent and using nothing but colourless fish and tiny glass beads inside.

A large apartment is not a prerequisite for a breathtaking aquarium. Tod Michael Volpe, an art appraiser and consultant who lives in a small studio in Murray Hill, has a five-foot-long, 150-gallon coral reef tank built into the divider between his living room and dining area.

“The tank is like my life force,” said Volpe, 61, a master scuba diver who got the tank last year. “It’s endless pleasure and satisfaction. I’ll stare at the tank until 2 o’clock in the morning.” While some aquarium owners don’t know the breeds of their fish, Volpe knows all the ones in his tank, as well as the individual habits of each. He has a poisonous rockfish he feeds by hand, a spotted purple grouper (“I brought him up since he was a baby, and now he’s nearly a foot long”), a “very wonderful” clown triggerfish and many others. He checks on the fish all the time to make sure they look healthy and he leaves the air-conditioner on in his apartment to keep the water cool.

Volpe was once a high-flier in Hollywood who bought artwork for celebrities like Barbra Streisand and Jack Nicholson, but in the late 1990s he pleaded guilty to defrauding them and served time in jail. Today, he said, the aquarium is part of his healing journey.

“To me, this is not a fish tank,” he said. “It teaches me about life, respect, how creatures who literally have a world all unto themselves can interact with each other and how happy they are when they have what they need.”

On a practical level, Volpe does worry about the weight of the tank. Caparatta, who built his aquarium, assured him that the base was properly reinforced. “I have hundreds of gallons of water that could destroy the apartment underneath,” Volpe said. “Joe said, ‘Tod — relax.’ ”

Mark Collier, who owns Custom Marine Aquaria in Scottsdale, Ariz., once built a 30-foot-long aquarium into the floor of someone’s game room, which contained a pool table, big-screen TV and “water wall” that gave the illusion that water was cascading into the aquarium (it wasn’t). Total installation cost for the aquarium alone: $200,000.

To clean the tank, he had to dive into it, wearing a cord around his ankle that his partner could use to pull him out if need be. “I would basically kind of crawl through the aquarium and back myself out again,” Collier said.

The aquarium is no longer in use, he added, as a bank has since foreclosed on the house.