They can move over glass, but I don't believe they can walk on sand. So the island would only work if you have a sandbed. If its a bare bottom tank, then an island won't contain them. Basically, you have to have the island surrounded by several inches of sand on each side to contain them. At that point, the only way they could move to another area would be to completely release from the rock and free float. That does happen on occasion, but usually only if the BTA is super stressed. So far, all 4 of mine are completely contained on their island. They move around and touch each other, which is fine, but they can't get to any of my other corals.