Lets start at the basics, generally you need 30 hours for the majority to hatch. So at 20 hours, you've probably got less than 30% hatchout.
To remove the bbs, there are a few methods, some like using a light, and syphon (or turkey baster). with decent hatchouts, I generally allow the unhatched eggs (that sink) and bbs to flow out the airline - leaving the last inch or so (little more than hatched eggs) in the hatchery. I tend to go for pretty full hatchouts - with a low hatchout - you may want to use the syphon/baster method to avoid all the unhatched cysts.
Oh - and here's an address you'll want to visit:
http://www.angelfish.net/yabbse/index.php
Andy