Fees shouldn't be the only concern for investors when selecting an index fund, Rosenbluth said.
"I think investors should look beyond the expense ratio," Rosenbluth said. "The underlying index matters."
For example, iShares Core Dividend Growth and SPDR S&P Dividend both invest in dividend-paying stocks, but track different indexes. As a result, the iShares fund is up 5.9 percent this year while the SPDR fund is up 11.6 percent.
Poor performance by actively managed funds has caused more investors to move money into passively managed index funds this year.
A majority of actively managed funds across all asset classes failed to beat their passively managed counterparts over a 10-year period through June 30, according to a Morningstar analysis.
Lower fees from index funds are part of the reason investors are ditching actively managed funds. Last year the asset-weighted average expense ratio for passive funds was 0.18 percent, compared with 0.78 percent for active funds, according to Morningstar.
Fees for the most popular passively managed funds will continue to gradually decline as they gather more assets, Rosenbluth said.