is it a plug? if its not encrusting they more then likely used superglue as well. so in that case use a razor blade and at the base of the frag and the plug just slice it.
Also if u use the aqua mend which they sell at any hardware store. just make ur self a lil mold to place the frag in just big enough then let it dry. once its dry about 30 mins of so put a small amount of superglue on the aqua mend and then place the frag in. the epoxy will sometimes mess with the frags base and burn it.
If you need any more on this let me know send me a message