US securities regulators announced Friday that Citigroup would pay $12 million for misrepresenting practices on a private exchange to clients, reports AFP from New York.

The Securities and Exchange Commission charged Citigroup with falsely leading users to believe high-frequency traders were not permitted on Citi Match, a premium-priced “dark pool,” a private marketplace separate from the New York Stock Exchange and other public markets.

In fact, two of Citi Match’s most active users were high-frequency traders and executed more than $9 billion in orders on the exchange, the SEC said.

Large investors like dark pools as a way of commissioning transactions without swaying prices and boosting the cost of their orders. But the venues have been

criticized for lack of transparency.

The SEC also said Citigroup failed to disclose that trades were routed to other platforms, including other dark pools.