Spaces are important, not only for the fish, but for the flow. the tighter your rockwork, the more dead spots you will have. I love the look of arches, pillars, etc. You can have fun with the design, but make sure it's very solid. You don't want a rock slide, so, use zip ties, putty/cement and drill where you can to use acrylic rods and you will be golden. Just for reference, here's my rock work before and after:
