New York Attorney General Eric Schneiderman filed a lawsuit last month, accusing the Barclays dark pool of giving high-frequency traders an unfair advantage, even though the bank had promised investors they would be protected from “predatory” and “toxic” traders. The lawsuit alleges that Barclays executed nearly all of its customers’ stock orders on its LX Liquidity Cross dark pool alternative trading system instead of on exchanges or other venues that might have offered better prices.