Spencer Platt/Getty ImagesNEW YORK - Zero is now the hottest number in the online brokerage industry. Last week, a smaller competitor, Interactive Brokers Group, started its new IBKR Lite service, which also eliminated commissions for US-listed stocks and ETFs. Shares of all four companies were hit hard in the past two days as investors worried about the impact that zero commission trading would have on their profits. E-Trade's move was all but inevitable after Schwab and TD Ameritrade each announced that they were ending commissions for investors trading stocks, futures and options. The entire discount brokerage industry is dealing with brutal competition from upstarts like Robinhood, which already offers trading services with no or low commissions via popular mobile apps.