I will be using bio spira for the 3rd time tomorrow. Essentially it's the bacteria found in a cycled tank. You may have ammonia and nitrite spikes, but not enough to cause harm to your fish. Here's some information and comments from other sites.
http://www.fishprofiles.com/interactive/forums/thread.asp?id=20043
http://fishgeeks.com/index.php?name=PNphpBB2&file=viewtopic&t=7372
http://www.fishprofiles.com/interactive/forums/thread.asp?id=20043
http://fishgeeks.com/index.php?name=PNphpBB2&file=viewtopic&t=7372