Hex tanks are a bit of a challenge due to the small foot print. One's I've seen that look nice have as central rock structure (tower) and maybe a few small pieces strewn around for interest. Fish need to be small, rock hugging species (clowns, basslets,gobies, firefish, cardinals) that don't require a lot of lateral swimming room. As long as your lighting is sufficient, your selection of corals is up to you. Some hex tanks are deep so make sure you have the light penetration.