I also use BluTak, a.k.a. "poster putty". I like it a lot because it's cheap, easy to move, and semi re-useable. As Bitsy said, it does lose it's adhesive qualities after a little while, but again, it's pretty cheap to replace. One pack of it can go a long way! It holds the probe in place very well for normal activity around the viv, and I don't have to worry about anybody getting stuck and losing scales on it. I do check it regularly, just to be sure it hasn't been forced out of position, but no problems yet!