The Securities and Exchange Commission has charged New York-based brokerage firm Linkbrokers Derivatives with unlawfully adding hidden markups and markdowns to customers’ trades to secretly take a profit of more than $18 million.

According to the SEC, between at least 2005 and February 2009, certain employees on Linkbrokers’ cash equity desk perpetrated a fraudulent scheme to charge customers false prices with markups and markdowns hidden within them. The fraud involved more than 36,000 customer transactions, the SEC said.