Bacopa monnieri
B. monnieri has a bright green leaf that is rounded. It will grow both inside and outside. It will bloom regularly inside or out, if it's allowed to grow out of the tank. The blossoms are small and white as you can see in the first photo.
My Bacopa tends to like to roam. It starts as an innocent looking little plant then proceededs to take over a large section of the fish room. The photo below was just the beginning. It eventually went into 7 other tanks and reached down to the floor. I've since pulled that batch out, but it's spread several times since then. One spring the Bacopa had some caterpillars in it that I thought would come out as nice butterflies but all I got was caterpillar holes in leaves and caterpillar poop on my shelves and in my tanks. I never found cocoons or butterflies and the caterpillars eventually disappeared.
For more information about Bacopa please see my February 2006 column for TFH's "The Planted Tank," called The Stemmed Plants.