You can't, that is the potential "problem" with anemones. They can get up an move. Corals can be placed certain distances apart to keep them from waging war against each other, but an anemone can up and move right through, stinging things in its path. Some reef keepers don't put anemones in their reef tanks, and others risk the chance. Basically, that is what it comes down to, IMO. Do you want to take the chance.