Can i keep a snake in a tank that previously had fish in it?

yes you can keep a snake in what used to be a fish tank. aslong as you disinfect the tank correctly - like you said without harsh chemicals- but it has to be properly cleaned. you may have used what you thought was not a harsh chemical. also, i would never trust treated wood in any of my reptile cages. pet shops will tell you anything to get you to spend your money. it is possibly stress related, that caused the snake to die, but i wouldnt say that was the only problem.