Any aquarium owner knows it's risky putting too many fish in one tank. On a larger scale, Tampa Bay may soon test the limits of how many major aquariums this area can support.

The Clearwater Marine Aquarium — a modest-sized place that's home to a world-famous dolphin — is striving to build a much bigger, state-of-the-art home in downtown Clearwater.

It would be 25 miles and 40 minutes away from the Florida Aquarium in downtown Tampa, a well-known attraction that survives because Tampa taxpayers subsidize it.

Could both of them thrive?

The Tampa Bay Times studied the country's most-visited aquariums and spoke to experts in the field, and found it is rare for two large aquariums to coexist in the same metropolitan area.

Count industry expert David Robison among the skeptics.

"As a tourist, I'm coming down there to do a variety of different things — the beach, the Dalí Museum, Busch Gardens, an aquarium. But I'm not going to spend the money to do two aquariums," said Robison, a LaSalle University economist who specializes in entertainment venues.

"You end up splitting the business. They're going to share the market, and that makes it difficult to survive."

Not so fast, say officials with the Clearwater aquarium. It's not that simple, they say.

"First, we have both been in this market for many years. This will not be a new issue just because we open a new facility," said David Yates, the Clearwater aquarium's CEO.

Yates rejects the notion that the two aquariums will cannibalize each other. He notes that attendance at the Florida Aquarium has remained steady even as crowds at Clearwater's aquarium nearly quadrupled after the 2011 release of the film Dolphin Tale. The movie is set at the Clearwater Marine Aquarium and stars its main attraction: Winter, the dolphin that learned to swim with a prosthetic tail.

"With our steep climb from 79,000 guests in 2006 to 750,000 in 2012, the Florida Aquarium has maintained the same general attendance levels," Yates said. "Thus, CMA has grown the aquarium market, not cannibalized the Florida Aquarium's numbers."

Still, the Clearwater aquarium's ambitious plan to build a $160 million, 200,000-square-foot attraction raises concerns. Its leaders predict that a flashy new place combined with Winter's fame would attract far more visitors than the current aquarium — possibly in the neighborhood of 2 million in its first year of operation. If true, that might not be good news for Tampa's aquarium.

Nobody knows for sure.

"If Busch Gardens opens a new roller coaster, it has an impact on us," said Thom Stork, CEO of the Florida Aquarium. "It could be negative. It could be positive because it brings more visitors to the market. What would be the impact of Clearwater's plan? I don't know."

The stakes are high. Major aquariums are expensive, with their round-the-clock utility bills and their need to keep thousands of animals alive. If this works out, Tampa Bay would have a pair of world-class aquariums in its collection of tourist destinations. If not, it could be saddled with two debt-ridden attractions competing for the same wallets.

To further complicate matters, there's actually a third aquarium in the picture. The 2,000-square-foot St. Petersburg Pier Aquarium has closed and is expanding into 13,500 square feet at John's Pass Village, the tourist district in Madeira Beach.

Renamed the Secrets of the Sea Marine Exploration Center and Aquarium, it intends to open in November. It expects 250,000 visitors annually — way more than the 62,000 it had last year.

A fourth local aquarium, the small Tarpon Springs Aquarium, has a 120,000-gallon reef tank and live feeding shows, but it tends to get visitors who are already touring the historic Sponge Docks district.

A rare case

Most large aquariums compete with zoos, museums and theme parks for tourists' attention. They don't compete with other aquariums. Of the 25 most-visited aquariums in the United States, the only metropolitan area besides Tampa Bay with two of them is San Francisco.

Around the country, some aquariums have flourished while others have struggled. Their stories offer clues as to how Tampa Bay's situation could play out.

San Francisco: In 1996, a for-profit aquarium opened only 6 miles away from the nonprofit Steinhart Aquarium, one of the oldest in the United States. The decision to build the new aquarium had been controversial, prompting 27 public hearings and seven lawsuits.

The new facility promised to pay the older one $200,000 a year for several years to offset its likely loss of visitors. But the Steinhart never collected a dime.

The newcomer, Underwater World, had a poor reputation and quickly got nicknamed "Underwhelming World." It went bankrupt and became a nonprofit, the Aquarium of the Bay.

Today the two aquariums coexist peacefully, occupying different niches. The Aquarium of the Bay focuses solely on the marine life of the San Francisco Bay, while the Steinhart's view is global.

The Steinhart is part of a bigger operation, the California Academy of Sciences, which also has a natural history museum and a planetarium. The Aquarium of the Bay is on touristy Pier 39 on Fisherman's Wharf, where out-of-towners come to gawk at sea lions, ride a two-story carousel and gaze out at Alcatraz Island.

"We appeal to different markets, and that's a big part of it," said Steinhart director Bart Shepherd. "Seventy percent of our visitors come from the San Francisco Bay area. I don't think a lot of those people go to Pier 39."

Tennessee: The 20-year-old Tennessee Aquarium is a success story, credited with revitalizing downtown Chattanooga. But it has faced growing competition.

"After the first five years, we leveled off at around a million visitors per year," said aquarium spokesman Thom Benson.

In 2000, when Ripley's Aquarium of the Smokies opened 150 miles away in Gatlinburg, it had a modest impact on the Tennessee Aquarium. Ripley's also draws a million visitors a year.

The biggest impact came when the world's largest aquarium opened in late 2005 in downtown Atlanta, only 120 miles away. Crowds at the Tennessee Aquarium dwindled by 30 percent, and 2006 was the only year it had an operating loss.

"Attendance dropped to about 700,000 after the Georgia Aquarium opened," Benson said. "Atlanta was our biggest feeder market."

In the years since, attendance has crept back up toward a million. Here, the Tennessee Aquarium's lack of debt was crucial. The place was fully paid for, so ticket revenue only had to cover operating expenses.

Like many U.S. aquariums, both the Florida Aquarium and the Clearwater Marine Aquarium are nonprofits that have to bring in enough cash to continue their missions of educating visitors about the wonders of marine life.

The 150,000-square-foot Florida Aquarium is known for its shark tank, stingray touch tank, penguin show, river otters, alligators, roseate spoonbills and 20,000 aquatic plants and animals. Its crowds are 60 percent bay area residents, 40 percent tourists.

"It's not Shamu, it's not Mickey Mouse," said Stork. People come because of the variety the aquarium offers, he said.

The Clearwater Marine Aquarium is best-known for its three dolphins and its rescue and rehabilitation of sea turtles and other injured marine animals. The place isn't glamorous — it's a former sewage treatment plant, with the old waste tanks turned into pools. The main dolphin area isn't air-conditioned and can be sweltering in the summertime.

Until recently, the Clearwater facility was a third the size of the Florida Aquarium. Now, following an expansion, it's half the size. It's also drawing bigger crowds than the Tampa aquarium. In fact, its attendance now ranks second only to Busch Gardens among Tampa Bay's attractions.

But there's no more room for the Clearwater aquarium to grow in its current location, a mostly residential community called Island Estates. At times, it has struggled to handle the crush of out-of-state visitors making pilgrimages to see Winter in the slippery flesh.

That's why aquarium officials want to build a snazzy new home on the downtown waterfront with a dolphin stadium, a two-story coral reef tank and a high-tech theater.

Heavy debt

The Florida Aquarium has one thing that the Clearwater Marine Aquarium doesn't have so far — tens of millions of dollars of debt. It was built with $84 million in bonds.

Tampa pays $6 million to $7 million a year in debt service for the aquarium and will do so until 2024. The city also subsidizes the aquarium's operations to the tune of $430,000 a year — down from a million dollars 10 years ago.

The Tampa aquarium's first expansion since it opened 17 years ago is under way. Through its $15 million Rising Tides capital campaign, it just built a new conference facility and new stingray pool, to be followed by new classrooms, a new exhibit hall and a 600-seat banquet hall to bring in more revenue from weddings and other events.

"My intention is to eliminate the subsidy. We are operating this aquarium as a business," Stork said. "We've been able to keep attendance at a level where we can continue to grow the aquarium. We will continue to do that, whether there's one other aquarium in the market or 10 other aquariums."

The Clearwater Marine Aquarium has significantly less debt. It owes $4.5 million on a loan and is considering borrowing another $10 million to finance improvements to its current home, which will be retained as a marine animal treatment and rehab center. However, aquarium officials predict they will have to borrow $60 million to $80 million of the $160 million cost of building a new aquarium downtown.

That gives some experts pause.

"I wouldn't care if there were no aquariums nearby. You shouldn't take on the debt," said Debra Kerr, who was executive vice president of Chicago's 83-year-old Shedd Aquarium for 17 years. "More than the competition, the biggest issue is the debt."

The luckiest aquariums were blessed with wealthy benefactors who wrote big checks to pay for construction. The founder of Hewlett-Packard built the Monterey Bay Aquarium. Home Depot's co-founder built the Georgia Aquarium. A family fortune tied to Coca-Cola paid for most of the Tennessee Aquarium.

But aquariums in Denver; Camden, N.J.; Long Beach, Calif.; and Tampa ran into financial trouble because they were saddled with heavy debt service on top of operating costs. The Florida Aquarium's overly rosy attendance projections led to problems soon after it opened in 1995. The city assumed responsibility for the aquarium's debt to keep it afloat.

Most of those aquariums "were startups with no history of success," said Yates, who's confident that a new Clearwater aquarium would be able to pay off its debt. "That is not the case with us. This is simply a new facility for an existing, globally known aquarium."

He says the aquarium could pay off $70 million of debt if its new facility got 950,000 visitors a year, which is only 200,000 more than the current small aquarium attracts.

In public forums, aquarium officials have assured Clearwater residents that taxpayers wouldn't be on the hook if the aquarium couldn't make its mortgage payments. In a November referendum, Clearwater voters will be asked to give the aquarium a lease for city-owned waterfront property. The terms are still being negotiated, but Clearwater, a smaller city than Tampa, would be under no legal obligation to take on the aquarium's debt.

If Winter dies?

There's an indelicate but obvious question that keeps cropping up at those forums: What happens when Winter dies?

Aquarium experts wonder the same thing.

"That's a lot riding on one dolphin," said Shepherd, the Steinhart Aquarium's director.

"That dolphin is not going to be around forever," said Robison, the LaSalle University economist.

Bottlenose dolphins can live into their 40s, although it's more common for them to live into their 20s. The Clearwater aquarium's previous star attraction, a beloved half-blind dolphin named Sunset Sam, died of liver failure at age 21.

Winter is 7 years old, gets the finest veterinary care and is reportedly in good health despite her lack of a tail.

Yates says another of the aquarium's rescued dolphins, Hope, will be featured in a planned Dolphin Tale sequel. He also thinks a state-of-the-art aquarium with diverse offerings will attract visitors on its own, minimizing the reliance on Winter's fame.

"People have been going to Orlando to see Mickey Mouse for decades," said Frank Dame, the aquarium's executive vice president, "and Mickey Mouse was never alive."

Times researcher Natalie Watson contributed to this report. Mike Brassfield can be reached at brassfield@tampabay.com or (727) 445-4151.

This story has been amended to reflect the following correction: The Clearwater Marine Aquarium is negotiating with the city regarding how much the aquarium would pay to lease Clearwater's City Hall property for a new aquarium if voters approve. An article on Sunday was incorrect on this point.