I have my baby albino checkered in a 20 gallon but it has a slide lock lid...that's why I want to look at your lid. A 20 gal is not too big for a baby...size is not the concern...escape and things the baby can get trapped under or hurt on would be the concern. Garters are masters of escape and babies are even better at it. They can get wet and then inch up the glass and find a way to wedge under gaps in the lid and get out, or worse, hurt or kill themselves getting trapped or smushed in a failed attempt. Wire lids have little holes on the corners and they don't lock down...but there are things you can do to rig it. If you have the money I can tell you of a very safe terrarium with a lock lid that you can buy at a pet store that will guarantee no escape for babies...or you could use a plastic tub...some people do that for babies...or we can work on rigging the lid of the tank on Craig's list.