Breeding tropical fish requires patience," said Hung yee Chung, who has ministered to the many varieties for 20 years. "One must understand the temperament of the different types of fish. When they become ill, one must apply medication, and when they are ready to lay eggs, one must separate them from the others."

Hung's fish farm sits on a roadside in the New Territories of Hong Kong, now the leading world supplier of the colorful creatures that are rapidly populating America's growing number of fish tanks. Trees surround the farm. As I walked through the gate, I saw four small children poking their fingers into the shallow fish ponds. The tensions of a two-hour hectic drive from Hong Kong's urban center dissipated among the trees, the laughter of the children and ponds full of gold fish: Those apparently are among the secrets of the business.

"Keeping a tank of tropical fish at home can soften one's temper," Hung said.

Last year, Hong Kong exported $3 million in tropical fish to the United States, almost a tenfold increase compared to 10 years ago, when Hong Kong exports to the United States totaled only $320,000.

Hung led me from his bare office to the indoor breeding room, near his family's kitchen and an open storage area where packaging boxes and tanks of oxygen are kept.

Inside the room are rows of water tanks. Some tanks are filled with baby fish; others with mother fish ready to lay eggs. One sad-looking gold fish moped in a separate tank. He was sick, I was told, and undergoing hospitalization.

The long-finned Angel fish in black and white stripes and the various types of tetra are particularly popular with American importers. Almost 10 years ago, merchants here began breeding their own fish by importing parent fish from Brazil and South East Asia. At present, almost all the imported fish - close to 100 different types - are bred in Hong Kong.

The temperate weather here has helped the industry. Also, Hong Kong has a year-round supply of tiny, tasty worms the fish love. Tropical fish grow to about 7 months old before they start reproducing. Some fish produce their young alive; others lay eggs on rocks or on weeds. Some betray a maternal instinct by secreting saliva around their eggs to protect them against predators. After 24 hours, baby fish will hatch out from their dark cell.

"The popularity of tropical fish is increasing," said exporter David Lam. "A tank of tropical fish not only adds delights in the homes, it also helps to break the monotony of watching television . . . it can help stimulate children's interest, motivate them to read books on fish."

Lam has an idea about the growing popularity of tropical fish in the United States, which has visited frequently. "The more industrialized a country is, the more its people are feeling isolated. They lose trust in each other and they no longer share feelings among themselves. They'd rather keep a pet and project their feelings on them. And so tropical fish are in demand."

About 700 fish farms are located in the New Territories. Most of them, like Hung's, are managed by individual households. Though there are a small number of modernized farms, Hong Kong remains very primitive in cross-breeding resarch and systematic production.

The average farm consists of an indoor and an outdoor breeding ground. Depending on the kind of fish, some are kept indoors in large glass tanks with temperature control while others are kept outdoors in shallow ponds. Indoor water is changed every week, outdoor water every two or three days.

Exporters receive orders from buyers usually via different airlines, and then package the fish in plastic bags. The bags are one-third filled with water, and pure oxygen is pumped into the rest of the cavity, then packed inside a paper or styrofoam box about 20 by 14 by 14 inches. A few drops of chemical keeps the water pure. With adequate oxygen, the fish will normally remain alive for 48 hours.

The wholesale and retail price of tropical fih varies a great deal. The neon tetra, a fish no more than one inch long with a shiny body of orange, greenish blue and gray, sells at $2 per 100 wholesale while retail stores in the States market it for 30 cents each.

Like much of Hong Kong industry, this is a low-cost, low-capital enterprise, yet risk is not totally absent. Closed away in their dark bags for the long plane trip to the United States, at least 5 per cent of the millions of fish that leave here each year die on the way, victims of too little oxygen and too many fish.