Dracaena deremensis 'Warneckei' are very prone to leaf spotting and tip browning for a variety of reason, some of which have been previously mentioned. The tip burning is likely caused by either over or under watering. This plant is not at all forgiving of even occasional or momentary lapses in watering.
Fluoride added to tap water is not concentrated enough to cause plant damage. However, fluoride in certain fertilizers with super-phosphate are serious sources of toxic fluoride. Hard water is a problem. If your tap water is hard, then use filtered, distilled, or rainwater instead.
