How many gallons should an adult (I'm uming) Lungfish live in?

Not getting one, don't want one, but a pet store I know of has one (that's not for sale, its the store pet/mascot, and they keep it in a tank (I'm guesstimating the dimensions here) that's 12 inches wide, 24 inches deep and in 12 inches of water. It can not turn around or move more than just swinging it's front half from side to side a bit. I would like to go an suggest they give it a bigger tank, but I don't want to go in there with no knowledge whatsoever.