People grew richer entering the 1970s. Not surprisingly, people started to look for some means of investment. The passing of Banking Ordinance of 1964 would begin the tightening up of banks. A minimum capital of HK$5 million and liquidity ratio of 25% and limitation on loans and investments became the new requirements to operate legally. People began to have more trust in their banks, and the accumulation of savings led to people's willingness to invest. Soon, the stock rush began. New stocks were on sale every day; brokerage houses mushroomed; some people quit their jobs to become full-time investors, suffering a kind of urban disease that doctors called "stock illness". It is said that the investors during this period numbered to 500,000. The Hang Seng Index kept soaring, but then crashed in March 1973, leaving many people bankrupt. The Hong Kong economy recovered only slowly over the next few years. In 1976, a "Deposit-taking Companies Ordinance" was also passed to enforce non-licensed bank institutions to register with the government. A minimum paid up capital requirement of HK$2.5 million was also required. The strategy was to mirror the Deposit Trust concept in the United States. As a result of the water shut-down by mainland China during the Cultural Revolution riots in Hong Kong, the government responded with a desalination plant at Lok On Pai, Castle Peak in 1975. The High Island Reservoir construction went from 1969 to 1979, and was expected to be the same size as Plover Cove. The construction, contracted by a Japanese company cost more than HK$400 million. Tsang Tsou Choi, or the "Kowloon Emperor", began his career of calligraphy graffiti in 1970.