I need to double check my husbandry of corn snakes

[Smilts]

I bought a 13 inch (dont know the weight) snow corn snake from the pet store in the town next to me and tonight it died. I would like to double check my husbandry to make sure it wasnt my fault. I dont need mass criticism if Im wrong then plz tell me. I kept it quarantined from my other healthy snakes in a 7qt sterilite locking lid tub. The top of the tub had lots of holes drilled in it, I lined the inside bottom with unscented paper towel 3 layers under 1 half of the tub I had a heating pad, with a digital thermo I made sure that the tub floor during the day stayed at 83/85 degrees during the nite it would drop down to78/80. I purchased the snake on the 29th the pet store told me it had eaten that day 1 pinkie. It was slighly lethargic then I found mites on it a few days ago and got advice on here to use diluted Nix shampoo to treat it. I gave it one treatment completely changing out and cleaning its tub as advised. Im not sure if I did something wrong, it never went to the restroom and refused food 5 days after I purchased it, and I havnt offered since.