never release it into the wild!
once it gets too big you will either have to kill it or try to sell it to someone who can keep it properly.
where are you located? how big is it now? what's the biggest tank you have/ will have?
i had mine in a 29 (similar dimensions to the 20L) when i got him at about 4" and in about a month and a half i was getting ready to move him to a 125 because he was about 6-7" already. i was feeding him bloodworms about twice a day