I've been playing around with just about everything, for sps frags I have to use the reefplugs otherwise the frags get blown all over the tank because of high water flow and then you get the whole sting wars thing when they start touching.
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I've also cutup several pounds of tonga branch into 1/4" slices, these make very interesting frag bases as they are not perfectly round and the corals seem to love this as a base
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White marble, I going to start a project today that is complicated. Involves using white marble as the substrate for zoanthids, it's easier to grow zoa's on marlbe and when you need to frag them you simply scrape them off and then frag them onto whatever base you like, same with my collection of colored cloves.
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I've also have several huge heads of decorative, dead coral, various types that I'll bustup and use as a frag substate