HN1 - Do your arithmetic, my friend. A 20 x 10 inch tank would have 200 square inches of surface, which means it could support 20 inches of fish, not 30.
Yes, air stones do agitate the water and circulate it, but not enough to compensate for lack of surface, where oxygen is exchanged.
Your assertion that improper feeding is the No. 1 mistake of aquarists is valid, but of course there are variables. Lets just say that overcrowding and improper feeding are the TWO most common mistakes, and leave it at that.