I've noticed you have far too many post about the exact same fish, try posting all your questions in the same thread as they come up, I am trying to help you understand your Bala sharks since I have owned mine for 4 years but I'm not really interested in reading a million and one differnt threads with almost the same question so this is what I'm going to do, listen up now cause this will be some advice that you may want to take head to......First off...
Bala Sharks can get up to a foot long maybe even larger, you should not cramp them into a 20g tank even if it is till they get a bit bigger because you will stunt their growth and with doing that they will not live as long because their outer shell has stopped growing but their insides have not therefore they pretty much suffocate on their own internal organs...
They can be very skitish and will not be aggressive at all, they will play with eachother slapping fins and chasing eachother around the tank, in order to help them get over their scaredness stay around the tank alot, move your hand slowly up and down the length of the tank to get them used to you and outside movement, as well they will start to follow your hand after a while because they will start to recognize you as the person who plays with them and feeds them, mine greet me at the glass everytime I come in the room now, but they will startle everyonce and a while and that is normal.....
I don't recommend having anything smaller that isn't bigger than their mouth cause even though they are herbavoires, they may accidently eat a small fish like a guppy or neon in their fedding frenzy....
As for being told they will take years to get over 6" that is a load of BS, start doing some research and listen to what the people on this site are telling you cause most of us have had fish for 5 or more years or had them before and are just starting out again....Your Balas could reach 6" in 6 months so upgrade your tank soon or you will be killing your fish.....
That is all I can think of right now but again start listening to us or you are just going to waste your $$$$