When picking out plants, look for variety. Oxygenating plants are essential to the health of your fish and therefore are a must. Both of these plants float below the surface. Surface plants are also a good choice. These plants float on the water and provide shade for the fish, keep the water cool, and reduce algae production.
Water lettuce is one variety that is available at most nurseries that deal in water gardens.
A nice complement to the surface plants are bottom plants such as water lilies. These bottom plants add interest and perspective to the lower regions of the barrel. Prices on these vary greatly, but it doesn't take an extremely fancy water lily to add dimension to your pond. To get the pot to the bottom without losing large amounts of soil, the surface around the plant should be covered with gravel or small rocks and then the plant should be slowly immersed. The plant does best when placed just off the bottom on a single rock or brick.