Okay...if it were me, I'd plant some of the Hygro on either corner covering a triangle area with the corner being the tip of the triangle. Then Id add the pogostemon stellata narrow leaf infront of the hygro starting at the center of the bottom of the triangles. Make sense? Then continue until they reach the back wall. Don't plant them in straight lines but in a pattern with one infront of the other than another in back. I hope I'm making sense. ??? Space them out so that they will fill in faster (I just got mine planted and am only telling you what I was told about spacing)
The other is a ground cover right?
So not to have everything uniform you should pick out a few green varieties of different plants and put them here and there leaving the center open to create an illusion of depth. Ya know, I am new at this, so only telling you what I would do.
I would use an Amazon Sword or a Crypt to put off center just behind mid-tank to aid in the illusion of depth.
What do you think?
It's a lot...but hey! You did