Schwab ETF OneSource on Tuesday morning announced it was doubling its lineup of commission-free ETFs to 503 in 79 Morningstar categories. The expanded lineup will include 90 funds from iShares, along with additional funds from 10 other ETF sponsors currently on the commission-free platform. These funds will begin trading with no fees on March 1.

Roughly an hour later, Fidelity Investments countered with news that it will expand its commission-free ETF platform to include more than 500 ETFs, up from 265 currently available commission-free. As of February 28, the platform will include additional ETFs from iShares, which builds on Fidelity's existing relationship with BlackRock, iShares' parent company. As part of the expansion plans, Fidelity said its platform in the coming months will provide more choice in the smart beta and active ETF categories from more than ten asset managers and ETF providers.

This is just the latest round in an ongoing price war among brokerages offering commission-free trades to increasingly fee-conscious investors. 

Of recent vintage, Firstrade Securities in New York made a splash last April when it began offering 703 commission-free ETFs on its platform. In July, Vanguard expanded its commission-free ETF lineup to nearly 1,800. And then last month, Bank of America offered commission-free trades on stocks and ETFs to more than 5.25 million customers enrolled in its Preferred Rewards program.

While ETFs charge an average of $4.70 per $1,000 invested, some that track broad U.S. equity indexes now charge as little as 30 or 40 cents. And Fidelity Investments’ move to offer free mutual funds has spurred speculation about when, not if, a zero-fee ETF will start trading.

According to Jordan Wathen, writing for the The Motley Fool in an August 10, 2018 post, Fidelity and other companies offering commission-free ETFs needn't worry. He said that although ETFs are essentially a loss leader—the equivalent of a Black Friday doorbuster deal—those companies have other means of recouping their lost commission on the trades, and customers may also decide to invest in other products for which a fee is charged.

Ultimately, Wathen said, investors shouldn’t believe that investing in a commission-free ETF is necessarily a better investment choice than one for which they are charged a commission. An investor’s goal is to make money by investing principal, not just to save on the cost to do so.