I'm not a fan of attaching coral fragments with zip ties. It is common but Earth-hostile. For some types of coral, you can attach them above water with various kinds of fixative, but that doesn't work for all species. Some projects use biodegradable materials like hemp cord, which should last long enough for the corals to attach to the frame. Other projects don't attach fragments at all, but rely on an attractive coating to make coral larvae want to colonize it. Comparing options is interesting.