On his days off, Bill Caldwell goes fishing at a huge warehouse in Arlington.

The manager of D.C.'s Cosmos Club is a collector of exotic fish, a hobby that takes him twice a week to Aquaria International, an unusual retailer of wholesale-priced fish and aquarium supplies.

Caldwell says he has spent hours gazing at the 224 fresh- and saltwater tanks that line the store's shelves. Recently, he also bought a 100-gallon tank and stand there for a saltwater set-up of his own. "The prices are reasonable compared to the competition," Caldwell explained, "and they have a gigantic selection."

Without the large sign out front that reads, "Aquaria International, Open to the Public," potential customers might pass by the warehouse on Shirlington Road, a building with an entrance that looks more like a loading dock than a door.

Inside, with its white cinderblock walls, 5,000-square-foot floor and rows of shelves, Aquaria resembles a hardware store more than a pet shop. Brothers Doug and Dave Shaw sell fish, ranging from the rare, saltwater Majestic Angel at $132 each to common goldfish, 20 for $1. Panther groupers and white-spotted puffers swim in the saltwater aquariums, while oscars and zebras lurk in the freshwater tanks.

The Shaws describe their business as "a wholesale-type concept in a retail store." They buy their fish directly from breeder-suppliers around the world, cutting out the middleman and allowing them to sell retail at wholesale prices. Doug, 30, said Aquaria may "sell a neon blue gourami for $5 a pair. A competitor sells them for $12 a pair." According to Doug, the brothers sell between 400 and 500 species of fish, accommodating everyone from serious collectors to pet seekers. Sales for the store, which opened last spring, topped $250,000 from April 1 to Feb. 15. The brothers estimate that level will rise to an annual rate of $500,000 by the store's second year.

Dave, 33, does the buying that allows them to sell at lower prices. Breeder-suppliers ship fish to the Shaws from Africa, Europe, South America, Hong Kong, Indonesia and Hawaii, according to Doug. The fish travel in bags of oxygenated water packed in styrofoam cartons, which are flown into National Airport, where the Shaws pick them up. While the brothers said they have never had a shipment lost, Doug said they occasionally receive three or four shipments the same evening and end up unpacking live fish late into the night.

Doug said that when he and Dave set up the store, they aimed to keep overhead down. By operating out of a warehouse, they pay one-third to one-half the rent they would for a store in a conventional retail area, according to Doug. They also cut costs by using centralized filtering systems. Aquaria has two banks of tanks, one salt- and one freshwater, each containing 112 aquariums. According to Doug, it takes about three hours to clean the filters and tanks, compared with the 30 hours it would take if they had bubble-up filters in each tank such as the ones in most fish stores.

The Shaws are as enthusiastic as their customers about the fish they sell. Dave was the first brother interested in fish and soon converted Doug to the hobby. Now Dave has one saltwater tank at home, an acrylic aquarium with cabinets. Doug, who sleeps on a waterbed, used to have seven tanks.

Dave was the first to expand his hobby into a job. He worked in a fish store while attending Columbia Union College in Takoma Park. Later, while working at the U.S. Capitol filing documents, he also held down a part-time job at Fish Ltd., a tropical fish store in Annandale. Six years ago, when his boss offered to sell him the store and finance the loan, Dave quit his job on the Hill and took up the fish business full-time.

A year ago, Doug moved from New Jersey to Northern Virginia, and the two pooled their capital, opening Aquaria International last spring. Doug said he used the money he had been saving to buy a house, and Dave used the money he had made at Fish Ltd. Together, the two had enough capital to get a loan.

"Dave had asked me a couple of times to be a manager, but I couldn't work for my brother. So he came up with the idea of a partnership. He couldn't do it on his own. The new store was too much for one person to run," Doug said. In February, Dave felt the new business was firmly on its feet, and he sold Fish Ltd.

Doug said the fish don't stay in the tanks for long. "We sell all the fish about every two weeks," he said. "Nine out of 10 Aquaria customers come in to buy, compared to a mall store where the majority of people just look.

"Buying fish is different from buying a dog. That's pretty much set," explained Doug. "You might buy another collar or leash or something. But with fish, well, we have one guy in here every day to see what we have in. He knows if he misses some rare fish in stock , he might not get a second chance. Collectors don't stick with the same fish. They keep putting new fish in and taking the old ones out." The Shaws also buy fish back from their customers, including fish the customers have bred.

David Kowalski, editor of "Pet Supplies and Marketing," which conducts an annual survey of the pet industry, said that Aquaria's stock of 400 to 500 species easily exceeds the average tropical fish store's inventory of 150 to 200 species. He also pointed out that the gross sales for fish and fish-related products per pet store averaged $70,000, well below Aquaria's projected annual sales of $500,000. Kowalski attributed Aquaria's success to two factors, aggressive pricing and a strengthening market.

After remaining level for five years, "fish sales have been going up again the last year and a half to two years," Kowalski said.

The discount price brings in some customers. "Here Aquaria they sell a fish for $2 that you would pay $5 for down the street," customer Caldwell said. For other customers, the wide selection of fish makes the store attractive. John Stieringer, of Vienna, comes to Aquaria for its "unusual selection." He collects such hard-to-find fish as gars, a slender, toothy freshwater species. He also said he likes dealing with the Shaws. "The guys that work here are real helpful. They know an incredible amount about fish, and they are always willing to cut you a good deal."