channel Cat

He's already getting too big for that tank, especially at the rate they grow and the amount of waste they create. Those things can grow a foot or more a year, maxing out at around 36" or so.
 
Grow em big, and eat em. Just kidding. Channels get huge it don't matter what size tank there in. Your going to need a heated swimming pool.:D Channels should never be purchased for a 20g, you really need to get him in a bigger tank if possible.
 
AquariaCentral.com