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Cut off the crown is safest way, including a small piece of stalk. Then you can frag into several pie shaped pieces and leave them on a bed of gravel/loose rock to heal. They should attach themselves and will heal in about a month. The stalk you cut should grow a new crown. If this is a very large toadstool, you can cut off an edge of the crown or cut a donut. Try to do this out of the tank. When you cut a toadstool (actually most softies), they release some pretty nasty chemicals. I accidentally "fragged" my toadstool and the zoas next to it immediately closed up and pointed in other direction. Anemone downwind closed from 15" down to a 2" ball. Xenia upwind didn't like much either.
make sure to use surgi gloves - unless you want a stinky hand. also - i think - the toady's have some toxic chemical like the zoos and paly's not sure how itense but keep it from shooting into your mouth and eye.
LOL stinky hand. Yeah all softies have that stench. The chemicals are fine so long as you don't eat it or get it in your eyes. I think xenia is the stinkiest of the softies though.
Definitely do it outside of the tank, even then be careful pending the size of the one you're cutting up in relation to water volume. I used to have a ton of different zoas in my 24g aquapod, after fragging my huge leather (in the tank, oops) and letting them grow out I lost ALL zoas in the tank, a few polyps I managed to get lose and move to my 70g recovered and are growing well. I think I had far to many leathers and healing ones for the water volume. I also almost lost my hammer, again once I moved it to my 70g it was not only fine but has more then quadrupled in size. Only thing that seems unaffected was GSP & rics.... leaving me pretty much a ric & toadstool tank LOL