A day after reprimanding a Chinese investment bank, Hong Kong's securities regulator hit Citi with a fine for misconduct at its dark pool operation.

Hong Kong's securities regulator fined Citigroup's securities arm HK$4 million ($509,650), the Securities and Futures Commission said in a statement on its website. The U.S. bank failed to comply with the code of conduct related to its alternative liquidity pool, also known as a dark pool, from December 2015 to August 2016.

Dark pools are a type of alternative electronic trading system that give investors the opportunity to place orders and make trades without publicly revealing their intentions during the search for a buyer or seller.

Dark Pool Access

More than 130 clients were allowed to tap the dark pool without being evaluated as to whether they were qualified investors, because of an incorrect system setting of client profiles. Citi has since taken action to rectify the issue, the SFC said.

The reprimand comes one day after the securities regulator reprimanded and fined CCB International Capital for failing to discharge its duties as the sole sponsor in the listing application of Fujian Dongya Aquatic Products in 2013 and 2014.