Well bta's are very nice nems......... The prob with them, like mentioned above they are movers...... and pack quite a sting, if there is anything in that tank that you cherish then don't put one in. They tend to stay put for the most part once they find a spot where they are happy with.
When I set up my new tank system, I set up a tank strickly for my bta's....... I have no worries now about them walking over my LPS/zoas now.