IMO, it is absolutely necessary to use RO/DI water for a ceph tank and highly desirable for inverts in general. RO/DI does more than lower the particulate matter suspended in your water. The pre-filters and carbon remove particulates and remove chemicals put in the water to keep it safe for drinking but are not all that good for you to drink (and can kill even freshwater fish). I know some areas of TX (and other states) use Chloramine which is hard to remove and may still show as ammonia in the water after RO/DI filtering. The RO component further filters suspended contaminants. The DI component (the "sand") attracts and traps metals/minerals (the primary reason DI water has no taste and bottled water producers add minerals back to their product). So, a lot of the "need" depends upon your water source but all government provided water will have, as a minimum, some type of bacteria killer that has to be removed.
