A Clearwater company has received certification from the Securities and Exchange Commission for its electronic stock-trading system. NexTrade said the SEC has given it the nod to begin trading equities immediately on its Electronic Crossing Network, which allows market-makers, mutual funds, pension funds and brokerage firms to post, cross and match orders for stocks. The company said its system has been certified to handle more than 300 million shares per day and plans to expand its capacity to 1 billion shares a day within the next few weeks.