1st - if you're using a tank for a baby, a 20 gal is recommended. An adult snake will be fine in a 40 gallon. If you do buy a 40 gallon and the snake is a baby, clutter the tank with fake plants, hides and/or crumpled paper so there is less empty space.
2nd - personally, I do not trust any commercial pet store. If you're going to buy a snake, it's best to get one from a privately owned pet store with a good reputation or from a breeder.
3rd - most people who use lighting in regards to heat lamps place them over the warm hide or create a basking area. I've always used racks, as ball pythons prefer cozy, dark spaces and heat lamps make humidity a real pain to control, so I'm not the best person to answer that particular question.
4th - humidity should always be in the 55% - 60% range, and a hydrometer is the best way to monitor this. When the snake is in shed, humidity can be bumped up just a bit until the shed is over.
5th - tubs are much easier to control ambient humidity and temperature. I use a rack system, so all of my snakes are in tubs.
No question is regards to the care of an animal is a dumb question
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