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The Frag-Mag Size Comparison
This is a product called "The Frag-Mag" by Aqua-Mags, LLC. (www.aqua-mags.com).
This is claimed to be a modular coral attachment system. I purchased them as part of a group by with my local reef club.
This is a magnet coral frag mount which you can move around as needed for example to help in adjusting lighting levels of new frags.
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The Frag-Mag Inside the Cap
I don't think this is supposed to open up but this is the back side which goes on the outside of the aquarium. This would be the dry side of the magnet.
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The Frag-Mag Gluing Surface
This is the inner part of the frag-mag where the coral fragment would be glued on. It is covered with some gray colored crush rock. I assume this is to provide a rough surface for gluing.