If you cut them and throw them back in the tank they will settle somewhere and setup shop in a calm area. If you want them to attach to a specific rock, put the cut shrooms of choice in a bowl with some small chunks of rock or the desired rock itself and cover it with netting. The netting needs to be something that will allow water to flow through, but keep the mushroom in the bowl. An example would be the nylon material that bags of onions are made out of or a bride's vale. Place the bowl in your tank and after a few days it will attach to the rock in the bowl. If you used small chunks of rock, the ones that have mushrooms attached can be glued to larger rocks if desired.
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