WUGONG LAKE, China -- Near the banks of the catfish farm he owns is Zhu Zhiqiu's secret weapon for breeding healthy fish: the medicine shed. Inside are iodine bottles, vitamin packets, and Chinese herbal concoctions he says substitute for antibiotics.

Zhu's fish farm, in a village on the lower reaches of the Yangtze River, sends 2.7 million catfish fillets each year to the United States through a Virginia importer. Despite his best efforts -- he has dozens of employees clearing trash from the water each day, and the fish are fed sacks of fish meal more expensive than rice -- Zhu's fish sometimes get sick. Then he brings out the drugs.

"It's standard practice," he said. "Everyone uses them to keep fish healthy."

Chinese exporters like him have seized much of the US market, accounting for 22 percent of all imports, because their fish are cheaper to raise.

The fish are being raised, however, in a country whose waterways are an ongoing problem, tainted by sewage, pesticides, heavy metals, and other pollutants. The situation is worse in the southern part of the country, where Zhu's farm is and where industrial runoff accumulates.

Like other fish farmers throughout the world, catfish growers in China turn to a variety of potions. But the extent to which they use traditional Chinese medicine, which cannot be tested for as easily in the Western countries that import fish, is unusual. Zhu says he uses only safe and legal drugs, but it was clear that some of his competitors have not been so scrupulous.

The competitors spike the water with banned substances to keep their farmed fish alive. Batches of seafood traded recently at the Shanghai fish market, for example, carried the tell-tale greenish tinge of malachite green, a disinfectant powder that has been banned in China because it is a suspected carcinogen but is still commonly used.

Illegal substances such as malachite green keep showing up in Chinese seafood shipped to the United States, provoking a partial US ban on such shipments earlier this month. It was the latest development in an ongoing global awakening about the risks of Chinese-made products, from toys tainted with lead paint to pet-food ingredients containing a deadly industrial chemical.

Using illegal disinfectants and antibiotics "is a lazy way of raising fish," Zhu said. "But it is extremely effective."

Many of the "Southern-style" catfish fillets on US grocery shelves these days are indeed from the south -- of China...

...Chinese imports make up about 5 percent of all catfish sold in the United States, but that figure is growing . In 2004, China sent fewer than 100 containers, at 20 tons each. By 2005, 200 containers were sent, and in 2006, 500 were shipped.