My biggest problem from relocating fish was bringing an invasive underwater plant (Eurasian Water Milfoil) to my pond. (at least I think that's where it came from)

Even tho I didn't put any water from the other BOW into my pond, it showed up.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).