A color dye usually used in concentrated gels for soft drinks was added to the water to turn it to a dark color. Flat and expired soda pop was also used for dark and filthy water.
The characters of the young Dahlia and Natasha Rimsky are played by the same actress Perla Haney-Jardine. This was supposed to show that Dahlia symbolically saw herself in Natasha as they were both young girls who were neglected by their mother (or in Natasha's case both her parents).
The heartbeat heard during Dahlia's migraines are from an ultrasound scan of this film's editor Daniel Rezende's wife, who got pregnant at the beginning of production.
(at around 8 mins) If looking closely during one of the elevator sequences, a manifestation of a ghost can be seen with Dahlia which is why Veeck tapped and scratched the monitor.
Water can be seen in nearly every single scene of the film whether it be rain water, dark water, humidity, or household appliances such as a toilet, bathtub, or a washing machine. Because water is in nearly every shot, producer Bill Mechanic has said that "Water is one of the lead characters in the movie".