Bill Carwile knows what makes a good guppy.

One of seven senior judges for the International Fancy Guppy Association (IFGA) and a best-of-show winner many times over, Carwile has been breeding guppies competitively for more than 20 years. What started as a childhood hobby became his ticket to the world beyond his rural home town of Rustburg, Va.

At 5 feet 4 inches, with white hair and gold-rimmed glasses, 61-year-old Carwile is the picture of a polite, soft-spoken Southern gentleman. He works as a quality control specialist for a concrete-mixing company in Rustburg, a few miles from Lynchburg. When he speaks, his accent rolls in warm Southern cadences. But he doesn’t speak often; a reticent, humble man, he rarely opens up.

Unless the topic is guppies.

He walks down the stairs of his basement, apologizing for the normal basement-stairs jetsam. “Normally I’m the only one that comes down here,” he explains. He threads through the darkness around old floor lamps and haphazard piles of boxes to a door on the other side of the room. When he opens the door, he is bathed in fluorescent brightness, accompanied by a tropical heat and a sharp, briny smell.

This is Carwile’s guppy room.

Dozens of five-, 10- and 20-gallon tanks line the walls; he estimates that the room can hold up to six dozen five-gallon fish tanks. He should know — he designed the entire thing after the first guppy room he built in another corner of his basement sustained heavy water damage. “I measured it and built it to get as many tanks in here as I could,” Carwile says.

A calm bubbling sound pervades the room.

Carwile had five tanks of guppies as a kid, and he wanted to breed the fish for competition even then. “That idea was latent for a long time,” Carwile remembers. Years later, when he was working long hours in construction, he would stop by local pet shops and think about his childhood ambition. Eventually, he bought a tank of fish, which promptly died. A replacement tank quickly followed suit. It took him three or four tries before he discovered the culprit: soft water.

“Most areas of the country,” Carwile explains, “the water’s perfectly fine for guppies. All you have to do is get the temperature the way they want it, and you’re done.” Even other parts of Rustburg have water that would be hospitable to guppies. But his house is on a narrow band of land that was once under the sea. The soft, acidic water has a low buffering capacity that makes it unsuitable for guppies.

The solution? What his guppy hobby friends laughingly call “Bill’s water treatment plant.”

Carwile leaves the aquarium room and moves through the dank basement to another door around the corner. Behind it is a room nearly identical in size to the guppy room. It houses a water purification system: two blue canisters resting atop a large tank, feeding water lines into the next room. He built the entire system, including a platform and stairs above the tanks and an ingenious draining apparatus.

“All of this is trial and error,” Carwile says as he heads back to the fish room. “Trial and error to get the water to where the guppies like it.” He has thought about moving, but treating the water is cheaper.

Carwile gestures to a guppy with a ragged tail and explains that the most common culprit in a guppy’s demise is a bacterial infection on their tails. Breeders will often trim the fish’s tail with a razor blade to keep it healthy.

“I knock ’em out with club soda,” Carwile says. Using a cup, he scoops the guppy out of his tank and fills the rest of the cup with the soda, which contains carbon dioxide. It knocks a fish out cold. He then moves quickly, spreading the fish’s fin with a tiny brush and slicing it with a scalpel. When he’s done, he drops the fish back in the tank. The entire process takes 20 to 30 seconds, and within a minute the fish revives without any harm.

Trimming a ragged, trailing fin also gives it a cleaner edge and triangular shape, important attributes in show fish. “It’s not illegal, but you can tell when one’s been razor-cut,” Carwile confides, “as opposed to a natural edge.” A natural edge garners more attention from the judges.

Color is an even more important quality in show guppies. There are dozens of different color classes, which include various mixes of colors — blue fish edged in black are one of Carwile’s specialties — as well as different patterns of color, such as spots and swirls.

He turns to a tank of smaller, bright green fish. “This is my best ace in the hole for color,” he says, adding that their unusual shade of lemon green is “the color you want.” Greens and blues — the colors Carwile breeds — tend to win best of show because, he says, they are genetically inclined to be big, “and big is better if everything else is right.”

Stan Shubel, author of “The Proper Care of Guppies” and “Aquarium Care of Guppies,” agrees. Shubel is the No. 1 senior judge for the IFGA, and he had an active role in creating the organization’s judging standards. Color intensity and size are important, he says, “if everything else is in proportion.” “Everything else” includes quite a bit; there are 100 points in judging a fish, including form, shape and symmetry of the body and fins. A prize-winning guppy will encapsulate those features as well as any fish can, though a truly excellent best-of-show fish would receive only 85 to 90 points. “We judge our fish against a theoretical perfect fish,” Carwile says, “and a perfect fish has yet to be and probably never will be bred.” Often the point spread between first and fourth place is only a point or two — sometimes much less.

The average male show guppy, from head to tail, is about 11 / 2 to 2 inches long, and female show guppies can be twice that length. They live for six to 18 months and eat brine shrimp, which Carwile also breeds; guppies being primed for competition are often kept in hotter water and fed more often to bring their size and color to the fore, but this shortens their lives.

Part of what Carwile finds fascinating about guppies is his ability to change how they look — partly by inbreeding, which concentrates the characteristics one looks for in a show guppy. Guppies withstand selective breeding very well, he explains. “People, not so well.” Of course, the best option would be out-crossing the fish with other lines of show guppies, but that’s becoming an increasingly rare opportunity for Carwile.

These days, breeding show guppies is a dying hobby.

Shubel cites the faltering economy as a limiting factor for hobbyists, but perhaps the largest obstacle to international guppy breeding is a recent restriction put into place by mail carriers against shipping guppies internationally. “You can bring fish into the United States,” Shubel explains in a puzzled voice, “but you can’t ship them out.” Breeders in the United States are not able to send their fish to other international hobbyists, which has had a dampening effect at home and abroad. “It’s a sad situation,” Shubel says.

“Our system of judging is what keeps us going,” Shubel continues optimistically. With the IFGA’s stringent standards and teams of judges that greatly reduce the chances of cheating, guppy enthusiasts strive to produce the best show fish they can.

“If they show their fish,” Shubel says, “they will get a fair shake at winning a trophy.” After all, anyone can compete; you don’t need to be a member of the IFGA to enter its shows.

The IFGA counts about 200 members, numbers that have remained relatively unchanged. However, historically, many more breeders would simply show up at competition, Shubel says. Ten or 15 years ago, they would see 600 to 1,200 entries for any given show; now they are lucky to see 500. The numbers of shows per year are also in decline; in its heyday 20 years ago, the IFGA limited the number of shows to 10 or 12 per year, but that’s down to six or seven.

Many of the men who helped Carwile get started have moved away, taken ill, died. It’s getting harder to find other breeders nearby, and he no longer belongs to a local guppy breeding club. The first two he belonged to broke up over the years; the third — the Chesapeake Guppy Club — meets hours away, in Baltimore, and Carwile is too busy with his day job to make regular trips to meetings.

Though he lives with Susie Joiner, his partner of several years, Carwile has never married and has no kids. Aside from his college years in Richmond, he has never lived outside the small town in which he grew up. He likes it here, and besides, the guppies keep him busy. As his friends tell him, “Keepin’ guppies keeps you close to home.”

Outside of his few remaining guppy-enthusiast friends, few people in Carwile’s life understand the allure of raising guppies. “They think it’s weird,” he says, good-naturedly, of the few work buddies who know about his unusual hobby. “But that’s okay. It is kind of weird.”

Carwile continues, and his voice gains force. “But it’s so many levels, to me. It’s water chemistry, the fish, the genetics, the plumbing and carpentry and electrical work, to have a fish room that works.” Raising show guppies is a detail-intensive hobby, and Carwile is a man who relishes attention to detail.

But for Carwile, breeding guppies isn’t about the fish, it’s about the friends. Paradoxically, it is the insular hobby that has opened up the world to him.

“I’ve got friends that I’ve had for 20 years that live all over the country. I’d never been on a plane before I got into this hobby,” he says.

A broad grin cracks his normally stoic face. “Now I go to fish shows every year.”