Some zoanthids grow into clusters when their colonies become large and can easily be pulled out several at a time. But if yours don't then you can still cut them off down near their bases with a sharp object. This won't really downsize the colony but it may stunt its growth in the areas that you cut. You can also place small pieces of rubble or frag disks in and around the colony and passively wait for the polyps to slowly grow onto them. Tweezing them off sort of works too but you'll do a lot of damage and in most cases you'll probably just rip the polyp in half before it pulls off of its base. Another option is to just start over again. Frag off a few polyps to keep and then sell the entire colony plus the rock that it is on and replace it with a new rock.
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