I had the same problem with my bubble tip. I got fed up eventually and tried the following experiment. I got one of my wifes glass bowls, a medium sized one. I took the rock I wanted him on and put him and the rock under the bowl. I made sure to leave a little room for current, but not much and only on one side. Well, he moved around that bowl for about a day, chose a spot on the rock and gave up. After three days, I removed the bowl and he stayed right in the same place till this day.
Like the subject of this post says, NOT GOOD ADVICE, but it worked.