Any suggestions for agressiveness?

I agree - 20 gallon long is going to really stress her out, being that she's only three weeks old - all this wide open space, and unless you are using the smallest plastic flower pot saucers as hides, she doesn't have small enough hides to feel safe.

I can post pictures of the tubs I use to set up babies if you'd like. Did you ask if she's even had her first meal yet?

Yes, shes had two meals. Shed once. Would it be possible to divide the tank in two?

Not really...most dividers would have to be adhered to the tank to keep them in place and aside from Silicone which would have to cure at least overnight there isn't really anything safe you could use.

Keep the 20L, it'll be a great tank for them later in life. Just go get a tub for now. They really are cheap. :)

This is the 12qt I have for my biggest juvi corn, but it gets the idea across.
http://i184.photobucket.com/albums/x...tub-inside.jpg

Hides should be either little flower pots cut down, saucers, or those coconut halves. Anything else would be too big for such a little one. And don't worry about gradients in a tub that small...they're still there as long as your heat source doesn't cover more than 1/3 the bottom.