Charles Schwab Corp. is imposing stricter limits on tax-aware long-short accounts, raising the minimum asset requirement for some separately managed accounts from $1 million to $10 million. Additionally, the firm will no longer enroll new clients or accept new funds into portfolio margin accounts, which utilize greater leverage. These changes, set to take effect on September 16, mark at least the third time Schwab has tightened restrictions on these accounts, following similar moves by rival Fidelity Investments amid concerns over the rapid growth of such complex, tax-optimizing strategies.