Although high current is also good, I agree with fishfreak. I think it is better to have them in a medium to low current area, because then you can actually see the pulsing rather than the current whipping. I have mine in a low current area and they are pulsing and multiplying like crazy. My wife loves watching the pulsing (and she convinced me not to move to strong current area).
Also, be careful with dosing iodine. If there is too much, it can have an adverse effect on xenia from what I have read.
I have observed that Kent's Coral Accel once a day seems to make the xenia fingers fan out like thick eyelashes. Very beautiful and healthy looking.
Finally, if you want to spread them throughout your tank, best way is to move a little LR piece of rubble next to a xenia that has two stalks attached to a main base. Then over a period of a week say, it will partially move one of the stalks over to the other rock. When it starts taking too long, you can complete the process with a razor, but not before at least half of the stalk is attached to hte new rock.
good luck,
kris