Two giant acrylic windows in Steve Hughes’s living room peer into his own reef. – Photo: Stephen Clarke

You have to repeat the phrase a couple of times to grasp the importance: the world’s second-largest, private, indoor, salt-water reef aquarium – and it is in Steve and Andrea Hughes’s Cayman Yacht Club living room.

The big question, of course, regards that No. 2 ranking: If the Hugheses own the second-largest aquarium of its type, what is the largest? The answer is fascinating.

The largest belongs to Bill Wann, who lives in Wisconsin and advises the Hugheses on design and construction, and built all their equipment. Mr. Wann says he is preparing to build another “largest, privately owned, indoor, salt-water aquarium,” although it may not qualify because it is so enormous that it won’t fit in his house. He is measuring his back yard.

Mr. Hughes admits to being slightly obsessed, but tells a compelling tale. The completion of the enormous tank, viewed through two, 9 feet by 5 feet, 2-inch thick, acrylic windows – sealed by high-performance silicone – in his living room, is the culmination of long, expensive and complex effort. Never mind the crowded chamber with its outdoor entrance, dedicated to pipes and pumps, regulators for aeration and flow rates, valves and filters for flushing and purification, and the 1,000-gallon sump tanks buried in the yard or any permits required by the Department of Environment.

To be clear, Mr. Hughes says, the world has larger aquaria, at the New York Aquarium and at San Diego’s Sea World. Then there’s the dual, three-story, 1,776 square foot Poseidon and Neptune Underwater Suites in Dubai’s Atlantis at The Palm hotel, where guests pay $8,200 per night to “sleep with the fishes,” 6,500 of them, behind floor-to-ceiling plate-glass walls, according to the hotel’s website.

Comparatively, Mr. Hughes’ aquarium is modest, but nonetheless ranks as a stunning accomplishment. It is 16-feet deep, weighs 20 tons and contains nearly 16,000 gallons of salt water, created in deep-water tanks on the property, then moved through a network of pipes, pumps and filters.

“Local seawater is pretty unpolluted, but I could not believe the crap from the open sea,” he says, referring to floating refuse and the problems of using “wild” water.

Mr. Hughes’ pumps move his homemade salt water into an enormous storage silo, where filters clear it, then send it into the tank. Filtration, aeration and circulation is constant, recycling the same water again and again, incrementally augmenting it with fresh-made supplies.

The tank recreates the ecosystem of a typical reef, and includes more than the handful of brightly colored tropical fish you might find in a hobbyist’s cuboid aquarium on a living-room table.

An aquarium artifical reef teems with activity, color and life. – Photo: Stephen Clarke

“You want to reflect closely what you’d see on a reef: fish, shrimp, rocks, crabs, tube worms, algae. It’s a living environment. It’s all alive,” Mr. Hughes says.

“The challenge is to get the balance right, so everything lives and grows.”

For example, he says, coralline algae requires calcium, “so that needs a proper balance.”

He hopes to get “a little mangrove growth, with extended roots over the edge” of the vertical rock formations. Specialty livestock can be selected from commercial outlets around the world, as are the fish themselves. “They’re little guys, no sharks, for example, which are big and dirty. We want to avoid predatory fish, so no one eats them, keeping those that are non-damaging to the environment.”

Still, not all marine creatures peacefully coexist, and Mr. Hughes says the tank is large enough that natural enemies can reside at opposite ends without incident, “and stay within their territories.”

The environment is so large and sufficiently diverse that the animals are unaware they are even in captivity, he says.

“Reef fish stay inside a table-sized area their whole lives. It’s big enough that they don’t realize.”

The size deceives them into thinking they are in the wild, which affects their behavior.

“I’ve never seen fish behave as they do in a large tank, For example, in Bill’s tank [Mr. Wann’s], they behave normally,” he says.

The population comprises angel fish, small parrot fish, butterfly fish, clown fish, damsels, invertebrates, crabs and denizens “at the bottom of the food chain. They make the tank stable so it matures and grows naturally.”

Brown/yellow algae marks the start of in-tank development, soon turning green, and finally pink, “of a type you want that won’t take over the environment,” Mr. Hughes says. “The light pink calcareous algae is what you want. You want that pink coating everything. Then it’s perfect.”

He pegs the cost of the project, “by the time it’s done, probably about $250,000,” almost half spent on the equipment, which is custom-designed, machined and assembled by Mr. Wann, and installed by both the Wisconsin aficionado and Mr. Hughes.

Mr. Wann, with a background in precision pharmaceutical manufacturing, says suitable equipment is not available commercially, and the $700,000 he has spent on aquaria through the years has largely been to build not only his own equipment, but also his own tools to make that equipment.

For example, he says, “I replace the filtration every couple of years” as he improves his own designs. And as his tools evolve, plans coalesce for a 60,000-gallon aquarium – more than three times bigger than Mr. Hughes’ – costing between $200,000 and $300,000. He hopes to complete it in the next two years. Mr. Hughes, owner of Hughes & Company, supplying essential oils to the global cosmetics industry, has been devoted since childhood to collecting and keeping fish in at least seven different U.K. homes, and helped operate a tropical fish store in Bedford for seven years.

He and his Brazilian wife moved to Cayman in 2014 and asked Phoenix Construction to design their home – and aquarium.

The architect, Mr. Hughes said, “thought, ‘yeah sure, about 3 feet by 4 feet?’ We said, ‘Well … no … about 20 feet by 9 feet by 10 feet.’ In architectural terms, it’s really nothing unduly complex. Much of the concrete tank is bonded to the main structure of the house.”

The Department of Environment’s Deputy Director for Research and Assessment Tim Austin worried about chemical additives, any discharges into the canal, pollution and purification, although Mr. Hughes’ 15,800 gallons is not remotely comparable to the effluent from a cruise ship. Mr. Hughes’ deep-water well for discharge resolved that concern.

“We advised Mr. Hughes through design and construction,” Mr. Austin said. “As far as I understand it, there were no official approvals needed for the installation other than routine planning permits.”

Because Mr. Hughes makes his own seawater, “he doesn’t need an extraction permit. The system is a closed loop – the water is recirculated and cleaned – so no discharge permit [is] needed.”

Agriculture officials approve livestock imports, he says, many of which are available “from the local pet trade, and no permit is needed for the take of local species that are not in some way regulated in Cayman, i.e., most fish over 8 inches in length.

“It’s basically no different to a large aquarium on your office desk – just a whole different scale,” Mr. Austin says.