because it's horrible stuff if you have...
I think it's really important to make sure you have the knowledge as to why things happen in your tank, otherwise you're left simply...wondering or worse still guessing. Then if it does disappear you need to know why because you've done something right. If you're wondering what it looks like - I've included a piccy below. As you can see it's pretty nasty stuff and will blanket your plants in a very short space of time.
How is it caused: Poor water distribution, low nutrients and dirty filter. So if you have BGA, slap on the wrist to you ;)
How to get rid: Very simple - 3 day black out. You see BGA is not actually an algae it's a bacteria that can photosynthesise so by cutting out the light it dies. Before you do your blackout perform a 30% water change, turn your lights off and put a sheet over the entire tank and leave for the full 72 hours. No peeking, just let the darkness do the work. When those 72 hours have passed (and your fish and plants will be fine during that time) you'll see your tank is sparkling clean and your plants fresher than ever. Once it's all done, simply do another 30% water change and then make sure it doesn't happen again (see how is it caused).
Of course for those of you who can't wait, you can treat with Easylife Blue Exit (great name heh!).