I've had Bacopa Monnieri in my tank for a while. I love the look of it, It's a great contrast to most other aquatic plants. It does grow straight up, with a few "branches" but they also head straight for the surface. 2 Plants in that space would look good. Although, it's easy to propagate so if you have the patience, one plant would do. I was growing it in less than 2wpg and it did fine. Now it's in 3.5wpg with pressurized Co2 and it grows fast, and even pearls a little under the leaves. Hope this helps.