A second green grouper tested positive for the cancer-causing chemical malachite green yesterday. Market sources said they believed the fish, taken from Aberdeen wholesale market, was from Thailand. Another freshwater fish sample - one of 13 checked yesterday - also tested positive for the fungicide. Mainland authorities said the sample came from a batch of fish which had been tested twice before export to Hong Kong. The government said more positive tests should be expected until sampling and inspection methods on both sides of the border were harmonised and 'the system of registered fish farms perfected'. Following the fiasco over the mainland's list of 18 accredited freshwater farms, some of which do not appear to exist, local fish traders have been asked to submit the names of their usual fish suppliers so they can be accredited by mainland authorities. Meanwhile, a Taiwanese fisheries official yesterday questioned the results of Hong Kong's test on Thursday on a green grouper from the island, which showed it contained malachite green. Hsieh Ta-wen, director of the fisheries department under the Council of Agriculture, said the grouper could have been contaminated by fish imported from elsewhere. 'We have long banned the use of malachite green in fish breeding in Taiwan and we are surprised to learn of such a report.' The first contaminated sample was taken from a fish stall at a Wellcome supermarket in North Point on Thursday. It was the first time malachite green had been found in a farmed marine fish. The findings followed a scare over mainland eels and freshwater fish tainted with the fungicide, which has now been banned in food. Secretary for Health, Welfare and Food York Chow Yat-ngok said yesterday: 'We traced the source [of the first contaminated grouper] to a possible fish farm in Taiwan.' He said more tests would be conducted on other marine fish in local farms and those being imported. Dr Chow also told the Legislative Council panel on food safety and environmental hygiene: 'We need to do a good job ourselves first in controlling imports. At the entry point and wholesale points we will do spot checks and collect samples.' Fish traders have been asked to retain sale records to enable the government to trace the source of fish. Imports are being accompanied by health certificates stating they are free of harmful additives. Dr Chow pleaded for time to set up a system to accredit fish farms. He apologised on Thursday for releasing the 'wrong information'. Of the 18 Guangdong farms listed, Hong Kong had records showing six had previously exported to the city, he said yesterday. 'We cannot [yet] be sure of [the accuracy of the list],' Dr Chow said. Hong Kong inspectors were sent yesterday to check the eight Shenzhen farms on the list. They will also visit the eight in Zhuhai and the other two, in Shunde .