I would start him off in the 6 foot cage and not bother with a tub. Retics grow like weeds. My SD went from 2 feet to 7 feet in about 8 months or so. And i would go bigger on that cage. I use AP T25s for my SD retic and big boas. They are 72x30x18 and i order it with a shelf too as they love to sit on them. Here's an old pic of Caesar chilling on his shelf lol. He was close to 7 feet there.