how do you keep zoa's from spreading in a BB tank

[kevensquint]

Hi , I have an sps tank that I am re-doing and would like to add a few colonies of zoa's. From past experience they end up spreading fast. I assume in a sandbed they would not spread. I do not have a sand bed. I was thinking of glueing small rock shelves against the back glass near the top. If the zoa's sat on those, they would not spread downward right? Any ideas?

 

[Fcwham]

If they are happy they will spread, the trick is just containing them as they spread. If you have some rock islands on a sand bed its easy enough to cut the ones that grow out into the sand. the same may be true against the back wall but I've had trouble getting them to be very happy on a vertical surface like the back of a tank.