(Note: If you're only planning to trade part-time, and you'll be focusing on swing trades that produce more profit potential than intra-day trades, then most pay-per-trade brokers will suffice. You don't have to consider changing online brokers until you've been formally trained.)
This amount is a bylaw imposed by the SEC. When you're intra-day trading stocks, and you're placing more than four roundtrip trades in any five-day rolling period, you're considered a pattern day trader. As such you must fund your account with no less than 25K. If your balance falls below that amount, you get hit with an equity call. This means you can't trade on margin or pattern day trade until you deposit enough funds to return your balance to 25K.
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