Money market fund yields may go even lower (unless they are already at zero). According to the Wall Street Journal
In recent days, yields on money-market funds have come under renewed pressure because of a new FDIC fee that makes it more expensive for banks to lend securities.
Unless you're using the money market fund as a temporary holding account for your trading, it makes more sense to have that money in a savings account instead. Savings account rates may be low, but they're a lot higher than money market fund yields. For example, Vanguard's largest money market fund, the Vanguard Prime Money Market Fund, has a current 7-day yield of 0.07%.