If tilapia aren't legal in your state, then the best herbicide option in my opinion is Sonar.
Sonar is a systemic herbicide which works over the growing season, often requiring 90 days for control. It is easy to apply, cost-effective, and ecologically safe. Just don't water your yard with treated pond-water...according to the label.
Since watermeal and duckweed are very small and drift around with the wind, it is tough to control (long-term) with contact herbicides like reward.
Sonar may give you more than one year of control. And Trent is right, microbes may help...post-treatment.
