Let's try to help him instead of telling the obvious - if it dies take it out.
First off, you said it had split. BTA's seem to split for 2 reasons: They have extra energy and are super happy, or the opposite, they are really stressed and it's a survival mechanism. Given your small tank (likely with fluctuating parameters), ammonia present, and less-than-ideal lights, I'd say it's survival mechanism. BTA's really shouldn't just detach from rocks and spend days floating around. When happy, they'll move a little bit, but usually not much. Letting go an floating is a sign that it's not happy. Not attaching is a sign that it's not finding a place it likes. It is entirely possible to keep BTA's alive and healthy in a small tank with PC's, but your other parameters need to be spot on, fresh bulbs (when did you change them last), the nem should be close to the surface. Even then, PC's usually don't seem to give the nem enough light. It seems BTA's are able to keep an anemone alive, but not thrive.
During your upgrade, what lights are you going to have? I'm assuming you're moving your rocks and sand too, right? If you are, be prepared for another cycle and make sure you monitor it so you can do water changes as necessary to you don't kill your inhabitants with the ammonia spike.
