I support your notion that it feels too symmetrical. There are two things I would recommend.
- Don't have both structures end at the middle of the tank. This will help break it up and not seem so symmetrical. I would make one of the structures extend beyond the center point of the tank. Then some dead space between the other structure.
- As mentioned by others, shorten one of the structures. Typically the longer structure, as mentioned above, will be the taller one. Make sure it's a noticeable difference in height.
I have my tank set up in a similar fashion for reference:
FTS by
Toby Broadfield, on Flickr