Pain while eating sweets and black layer on tooth is indicative of initial stage of tooth decay. That will require a simple filling. It is not caused because of water or any chemicals in tooth paste. Cause of tooth decay can be eating sticky food..or any other food that cannot be cleaned from tooth surface. Drinking plenty of water n rinsing mouth with water after eating anything would help reduce the incidence of decay. Brushing your teeth 2 times a day will also help in preventing tooth decay.