Problem with 1 gal tank

Question
My 3 year old son got me a small 1 gallon triangle shaped tank for Mother's Day.  Since I got it early, I set it up according to the manual that came with it and let it run for about five days.  The top of the tank filled with air bubbles when the lid was on.  When I took the lid off, it stopped.  

Two days ago my son bought me two goldfish and they seem to be doing great.  Now, the tank bubbles almost all the time-with the lid on or off.  It's causing the water level to slowly sink.  When I add food to the tank, the bubbles disappear.

Is this normal?  Is there anything I can do to stop the bubbles?  Please help-my son is so excited by the present and I don't want to disappoint him.  

Answer
Hello Tricia-

By bubbles, do you mean tiny little bubbles that stick to the sides of the tank, or big, foaming bubbles (like if you were blowing through a straw?) The bubbles shouldn't cause any harm to the fish, it's probably doing them good by oxygenating the water. Be sure to check for any leaks in the seams of the tank, and feel free to write back (my e-mail is abworman@purdue.edu)

Hope this was helpful!
-Amber Worman