20L is not to big. If that's what it is. Gravel? I'm glad you have him. He should have two hides. One on the cool side one on the hot side. Take a look at the pinned or sticky threads on how to properly set up a glass tank. He might need more time to acclimate to his new home. And a hiding snake is a happy snake. Make sure your rats are proper temp at feeding. Some are extremely picky eaters. Also look at the my snakes not eating sticky and hides my way sticky.