Welcome to our seventh weekly fund review, in which we review the past week's notable fund news and tell you what it means for Foolish investors.

ProShares benefits from investor jitters
Apparently, investors are getting nervous about a potential market downturn, and bear-market ETFs are reaping the benefit. Assets at exchange-traded fund provider ProShares have grown feverishly over the past year. After entering the ETF market last June, the firm now boasts almost $6 billion in net assets. ProShares currently offers 52 different ETFs, roughly half of which short the market. These ETFs, including the UltraShort QQQ ETF (AMEX:QID) and the UltraShort S&P 500 ETF (AMEX:SDS), are designed to produce positive returns when the market falls. In recent weeks, these funds have seen a marked increase in inflows and in trading volume, as investors increasingly worry about an imminent market downturn

With a market correction likely near, it may be tempting to invest in one of these bear-market ETFs. An UltraShort ETF could potentially hedge against existing long positions in the market. But it's nearly impossible to be a successful market-timer, and trying to exactly predict a downturn is a losing battle. Investors should instead consider shifting their portfolio from recently high-performing areas into relatively lagging sectors. Remember, the market rises far more often than it falls, which means that these funds will do just the opposite.

Fidelity launches Income Replacement Funds
Fidelity plans to launch 11 funds that will convert investors' assets into regular payments. These funds will seek to make payments through the end of each fund's target date, ranging from 2016 to 2036. Each month, shareholders will receive the dividends from the Income Replacement Fund. If that amount falls short of the monthly payment, shares of the fund are automatically sold to meet the shortfall. Fidelity stated that these funds were created in response to increased demand for products that convert accumulated assets into a payment stream.

If your main investing concern is a required monthly payment every month, these new funds might be attractive to you. But there's no reason why anyone couldn't calculate their own required monthly payment on their own, instead of paying Fidelity to do it. However, these funds' creation suggests some degree of investor demand for such a product. If you decide this type of fund is right for you, look at the experience of the management team, the investment process, and the overall fund operations before buying in, and ensure that the fund makes strategic sense for your portfolio.

Two more ETFs you never knew you needed
In further evidence that the ETF steamroller rumbles onward, Van Eck is introducing two new exchange-traded funds focused on narrow market segments: nuclear energy and farming. The nuclear fund will invest in companies involved in uranium storage and enrichment, nuclear plant construction, and nuclear energy generation, such as Exelon (NYSE:EXC). The agriculture ETF will focus on companies involved in agriproducts, livestock, and agricultural equipment, such as Monsanto (NYSE:MON).

Why would any typical investor need direct exposure to either the nuclear or farming industries? I'm guessing the vast majority of assets these funds get will be purely speculative. People who don't know better will buy, thinking they can outperform the market by investing in such a narrow slice of it. Don't fall into this trap. Even more of these specialized ETFs will doubtlessly hit the market in coming months, and I'm betting you won't need any of them. Keep your focus broad, and remember that diversification is the key to long-term success.

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Fool contributor Amanda Kish lives in Rochester, N.Y., and does not own shares of any of the companies or funds mentioned herein. The Fool has a disclosure policy.