I see what you're saying. At a second glance the big rock really is placed awkwardly. I was too obsessed with trying to create a triangle that I didn't notice the obvious.
I've actually taken a liking to the aquascapes that have a lot of open space up top with a beautiful carpet and shorter plants in the mix. No huge plant formations hindering the negative space.
I'll do some modifications tomorrow evening and post results here. Please check back and take a look! Also, all additional comments would be greatly appreciated!