An alternative (especially for the Colts) is a small Tupperware type of container (the ones I use are 2 1/4 inches across and high) You stick the frag in, and put some rubble around it. Put it in a lower flow area, and in a week or two (depending on the species) the frag will attach to the rubble. I have done this with the hopeless Colt, Toadstools, and the leather from Kari with complete success.
