I've used lemna minor.
"Duckweed" is a bit like "carp", a loose terms encompasing a large number of different species. Duckweed is the common name for the group Lemnaceae, containing around 40 different species of Wolffia, Lemna, Spirodela and Wolfienna. Wolffia are typically the smallest of the lot, and Spirodela the largest. I use fat Duckweed or Lemna gibba, therefore my observations may be biast towards these guys. The size of the frond (leaves), length of the roots, and number of the fronds on the little plant is highly dependant on the nutrients in the system, growing conditions and age of the plant. With a lot of nutrients around, the roots can be quite small, as is the case with a fast-growing colony. An older colony, or a colony in low nutrient water can have very large root systems.
All of these are nice food sources for tilapia, but you may have to grow a lot to keep a tank full of fish happy - remember that around 80 - 95% of the duckweed is water.
Do you know where is Jamaica I can get duck weed?