Hard water doesn't have to mean harsh hair, Learn how:

Accumulation of mineral scale on surfaces due to hard water build up is an unfortunately common and truly aggravating problem. Most people have experienced the joys of living with hard water: cloudy, spotty dishes coming out of the dishwasher, diminished performance of coffeemakers, clogged or broken pipes and washing machines with an unpleasant odor that don’t work properly, turning clothes and towels dingy grey or a rust-tinged color.
Hair is susceptible to this menace as well, becoming dull, limp, or frizzy and more prone to tangles and hair breakage due to accumulation of minerals causing hair build up. Certain strong shampoos, such as clarifying or chelating ones, are marketed as solutions to some of this, but are there any options for those avoiding sulfate-based surfactants? As always, the answer to that question lies in the chemistry and materials science of the system.
Why Hard Water Creates Hair Build Up
Hard water contains significant quantities of dissolved minerals, such as iron, calcium, magnesium, and silicon. These metals can react with substances in soaps and shampoos and reduce the effectiveness of those products’ cleansing properties, making it necessary to use more of the cleanser. But, even more disturbing, is that fact that the reaction products precipitate out of the solution and deposit onto the surface of your hair, where they bind with the negatively charged surface. This is what is typically referred to as mineral scale, which conjures up a bit of a mental image, if one considers it. Picture hard fish scales covering your hair, creating a rough surface that prevents moisture from penetrating the hair shaft, and you won’t be too far removed from the reality.
These deposits also attract and trap organic matter such as grease and dirt. This leads to hair that becomes increasingly difficult to deal with. It becomes dull instead of glossy, loses curl retention capability, is more prone to formation of snarls and tangles and is more easily damaged. It can even lead to the development of an unpleasant odor to the hair, particularly in dreadlocks.
Clearly, this kind of hair build up is not a trivial issue and should be addressed as a normal part of a person’s hair care routine.
How to Prevent Hair Build Up
The absolute best method for dealing with hard water is to prevent hair build up in the first place. One can do this by utilizing a good water filter  that removes the unwanted metal ions from the water. Another technique is to use a chelating shampoo regularly, which has molecules in it such as EDTA, or acetic or citric acid. These acids bind with the metals in the water as you are washing your hair and are then rinsed away instead of depositing onto the surface of your hair. These shampoos can be harsh, though, and should always be followed up with a good conditioner, but even then, they may be damaging to curly hair if used too often.
Vinegar rinses can possibly help loosen mineral scale so it can be rinsed, and it definitely helps dissolve some of the trapped organic matter that can be lurking in the residue. Clarifying Shampoos can also help remove hair build up. It is not clear whether they actually remove mineral scale from hair, but they definitely can provide deep cleaning of any other matter adhering to the surface because of the mineral scale.
Chelating shampoos may be able to dissolve the mineral scale and help remove it from the hair. The Beauty Brains, a site run by cosmetic chemist/consultant Perry Romanowski, states their skepticism as to whether this works at all, which makes me yearn, yet again, for a lab with some really expensive equipment so I could run some studies, both to satisfy my own curiosity and so I could also give you all a definitive answer.
Gentle shampoos with surfacacnts designed to provide mild cleansing are undoubtedly capable of removing organic material and hair build up. This includes surfactants such as sodium cocoyl isethionate and coco betaine. However, it seems unlikely that these would have the ability to remove mineral scale by themselves. Fortunately, there are definitely some shampoos that contain mild surfactants, no added conditioning agents, and acids that are thought to aid in removal of hard water hair build up.
Since it is so evident that curly hair performs absolutely at its best when it has both a clean surface and a well-moisturized cortex, it seems imperative that you take some sort of measure to prevent or remove hair build up caused by hard water. The installation of a water softener or filter seems to be the best and most proactive solution. However, there are other alternatives, such as rinsing with vinegar and using chelating shampoos. It is of utmost importance that an excellent conditioner be used whenever one uses strong products like this on the hair, so don’t skimp on that step.