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. But for a certain segment of the population  many of whom never considered keeping fish before they had a big space to decorate  a showpiece aquarium has become a must-have piece of décor.

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?”

Image Karin and Alan Wilzig’s colorless fish pick up colors from lighting. Credit... Trevor Tondro for The New York Times

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.