Possibly purchasing a young Ball Python this weekend. Need some advice.

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Turns out the snake is around a year old. But I did figure that tank is overly large so I'm planning on getting as many hides as he needs. The current owner said since he's cold he's not growing that much in diameter. Could something like that be a major worry or just climate related? Anyways thank you for the helpful advice.

If he's not growing much, he's underfed. Proper temps, proper humidity, consistent feedings, and he'll start growing again.

If he's been kept in cold conditions, make sure he doesn't have a respiratory infection. Listen for wheezing, clicking, popping noises while he breathes, and check for mucus/saliva around the mouth.

If he's relatively healthy just underfed and kept too cold, once you restore proper conditions he'll most likely recover fairly quickly.