Neither do snakes! That's a myth! The only thing you'd be able to keep in a 20 gallon is a hatchling and even then, it depends on the breed. Also, requires a ton of investment in other equipment.
I advise strongly against a snake unless you've researched it and know what you're getting into.
Also, do you know where you're going to be in 20 years? Because a turtle or snake you get now will very likely still be around then. They are a huge commitment.
From someone who had no clue what she was getting into when she bought her cute little twoonie turtles to another. They're now over a decade old, quite, quite large and were still learning how to bond with each other.