I bought a beautiful coral from a LFS last week, and it had a plug that had a blue milk cap inside of it. I saw this before but thought nothing of it but this time it gave me an idea of what this is for.
I took a water bottle and cut the top off just under the thick ring.
then I used Aqua Mend to attach the bottle top to a rock.
Then I took the cap from the bottle, and covered it with white portland diy rock. The cap is flat to the cement.
Top view.
Then I screwed the plug onto the rock.
This is a great way to attach frags to the back wall.
By the way here is the coral I got from my LFS.
