Did you fertilize the plant? If you did, this may be a possible cause. Your plant really doesn't need any extra help other then a good quality potting mix.
Also if the water you are using to water your plant is softened or treated water (ie. florides) this could be causing the dark tips of the plant.
The salts sometimes build up in the soil and the plant becomes overloaded.
You can try using distilled water or even rain water. You also can fill a container of tap water and allow to sit 24 hours before using.
Here is a link to refresh you on the care requirements.