The race to the zero-bound is on. That’s not an interest-rate policy development, but an investing fee trend. Charles Schwab Corp SCHW, -2.41% on Tuesday said it would no longer charge a commission on trades of U.S. and Canada-listed stocks, exchange-traded funds, and options, starting October 7. That’s just the latest volley in the incredible shrinking broker fee war. Behemoth asset managers like Vanguard, Fidelity and Schwab have spent the past few years leapfrogging each other to offer lower and lower commissions. That’s thanks in part to the rise in popularity of ETFs, funds that in most cases passively follow...