Clean algae from zoa frag

[lastlight]

I've had two blue polyps clinging to a small piece of rubble for many months now. What started as a colony of 8-10 polyps was over-run with a thinly branching algae that weaves in between the polyps and chokes them over time.

I had the rubble sitting on the sand bed for a few weeks and it was too close to another small colony of 40-50 zoas. That colony now has a few thin green tendrils winding through it!

I have plucked nearly all of it off and it slowly comes back. Is there anything I can dip the rubble in that will kill this algae and not harm the remaining polyps? Thanks!