I'm in Florida. When a child turns 18, does the child support automatically stop or does something need to be filed to make that happen?

Child support terminates when the child turns 18, unless the child is still in high school, in which case it terminates at graduation, provided the child is set to graduate before the age of 19. Typically the new child support orders contain a termination date. Otherwise, you would have to file a Motion to Terminate Child Support (and vacate income deduction order), provided you are not just making child support payments directly to the other parent. For a more specific response, it would serve your interests to have an attorney review your order/judgment on the matter and advise you accordingly. All the best.