The stock market has taken a downward turn recently, thanks to fears about the coronavirus outbreak. And banks have been hit harder than most.

The short explanation is that interest rates have fallen dramatically as investors have begun to pile into "safe" assets like Treasury bonds. In fact, as I'm writing this, both the 10-year and 30-year Treasury yields are just above their all-time lows. Since banks rely on interest income to make money, lower rates could put pressure on profit margins.

While bank profits could absolutely suffer in the short term, now could be an excellent time to add some rock-solid bank stocks, like Bank of America (NYSE:BAC) and Goldman Sachs (NYSE:GS), to your portfolio. These two banks are both down 13% from their 52-week highs, despite excellent long-term growth catalysts and strong business results.

Marble entrance to a building with the word bank engraved over doorway.

Image source: Getty Images.

Impressive growth and a massive capital return

Bank of America has been one of the largest positions in my own stock portfolio for some time now, and I don't expect that to change anytime soon, especially after the bank's latest numbers. While growth has been rather slow at many of the other big U.S. banks, Bank of America grew its deposits and loans in 2019 by 5% and 7%, respectively. Not only is Bank of America doing a good job of growing in a weakening economy, but it's also growing rapidly despite the surge in online-based competition. In recent years, the bank has invested heavily in its technology and has greatly improved its efficiency.

In addition, Bank of America has done a fantastic job of returning capital to shareholders through dividends and buybacks. In 2019 alone, Bank of America bought back nearly 9% of its outstanding shares, and the newly reduced share price should help make the massive buyback authorization even more effective.

Bank of America is now the second largest investment in Berkshire Hathaway's (NYSE:BRK.A)(NYSE:BRK.B) massive stock portfolio, and CEO Warren Buffett has spoken very favorably about the bank. It's not hard to see why.

A big push into consumer banking

Goldman Sachs has historically been known as a bank for rich corporations and "the 1%." However, that's changing. One of CEO David Solomon's main priorities is to build out the consumer side of the business.

Goldman has already had tremendous success with its Marcus high-yield savings and personal loan platform. And the early results from Goldman's first attempt at the credit card business, the Apple (NASDAQ: AAPL) Card, have been impressive. In all, Goldman's consumer banking revenue grew by 23% in 2019 thanks to the continued success of these products.

However, this could be just the tip of the iceberg. Goldman already plans to introduce a wealth management platform that caters to everyday Americans, and its Marcus platform plans to offer checking accounts to complement its savings accounts. Solomon has mentioned several other possible consumer products, such as auto loans and mortgages. And here's something to keep in mind -- Goldman is pushing into consumer banking with one of the best brand names in the financial industry and without an expensive branch infrastructure to worry about. These two factors should combine into a big competitive advantage as it expands its consumer products.

Buy for the long term

As a final thought, it's important to mention that these could both be rather volatile in the near term. If interest rates remain near historic lows, or fall even further, profit margins (and bank stock prices) could suffer across the banking industry. However, these are both well-run banks with lots of room to grow, and investors should do quite well over the long run.