There are three main stages where water quality is of great concern to aquarium keepers.  They are from the tap water, pre-tank water, and in-tank water.  All of these have to be dealt with in one manner or another.  When talking about water quality problems in general one needs to know at which stage the water is at that is being discussed.  Often times this is obvious, but other times it is unclear.  Each stage presents it's own issues and problems.  The desired properties of the water, and how to get it to the properties that you desire, depends on the condition of the water at the stage it is in.  Just as there are at least thousands of different ways to keep aquariums, there are at least as many different qualities of water.  Knowing both the current state of the water as well as where it needs to be is key to getting the results you desire.

    Consider your tap water as the foundation of your water quality issues.  Everything that you will do to your water will be based off of what comes out of your tap.  No, this does not apply to those of you who get your water in bulk from Sam's Club, your LFS, or any other source.

    Now, before we get to involved here you need to know what is coming out of your tap.  Many aquarists suggest testing your tap water, and that is fine.  What you should understand is that your water quality from your tap will vary from time to time and season to season.  The most basic problem with testing your own water is that you simply have crude methods, at best, of testing.  Even if you get rather top of the line test kits you are still woefully out of the quality range of even the most simply equipped municipal water treatment plant.  

    All public water supply systems are governed by rather strict water quality standards and laws.  Many of these require your water company to monitor and report many major and several minor water quality parameters.  Since these water plants are public, they have an equal responsibility to offer reports to the public on these tests.  You need to start here!

    Bothering with researching specifications on at home water treatment equipment, or spending money on any of them without first consulting the reports on your tap water would be like preparing for a camping trip to an unknown location.  Virtually impossible!  I suggest you do not even try it.  I am well aware that in almost all locations there are other aquarists, people at the LFS, and those that sell waterpurification systems that will have very strong opinions about what you need.  Go ahead and listen to them, just do yourself a favor and do not follow them blindly.  Some of them surely have no real knowledge about what they are saying, recommending, nor even why they would use one system/method over another.  My opinion is that until you are quite sure what is coming out of your tap, have educated yourself a bit, and are sure you know what you need, do not buy anything.

    Many municipal water companies will have no trouble letting you look at the records for at least the past year, what your looking for is seasonal changes in readings, and the approximate times these occur.  If you can get them to give you access to the records for many past years you can get a real good idea when to expect what parameter to change.  This will enable you to tweak your filtration processes to match the change in tap water quality.

    Now that you have some idea about the known water qualities of the water in your area you can begin to come up with a plan to convert that water into the specifications that you require.  Since water varies by location I will just be going over the equipment that I employ and the reasoning behind it.  I will not go over every possibility, but hope that you can get the basic ideas by what I add here.

    Some troublesome pre-tank water quality issues can be traced back to the storage conditions of the water post filtration yet prior to being used in-tank.  Water is possibly the worlds most amazing solvent.  As such it can pull pollutants out of the air if the water is clean enough to begin with.  Also, most household air is not exactly as clean as most would think. This being so, even water that isn't all that clean, may pick up and pull things out of the air.  Some of these can cause trouble over time, some others are harmless for the most part.

    Here is the area where most aquarists try, often in vain, to control water quality.  Often times this proves to be a mistake.  Do not misunderstand me, I am not implying that in-tank water parameters are not important and should not be tested.  They most surely are important and should definitely be tested!  Yet my point is that I feel many spend to much time here when the actual cause of the problem is either in the tap water, or the pre-tank water.  On some occasions hard to detect issues with pre-tank water can multiply in-tank to the point where unavoidable problems erupt.  Generally when this happens it leaves the average aquarium keeper scratching their heads.

    An example of this would be a residual presence of either Phosphate or Silicate.  Over time, if these nutrients are present in the tanks top off water, they accumulate to the point that if corrective measures are not taken they result in an explosion of algae. The interesting part of this is that often times they are not measurable in the tank water itself as it is rapidly sucked out of the water column by various organisms and mechanisms.  This continues until maximum absorption and/or adsorption is reached, then they just dump this back into the system in one form of nuisance or another.  Now since many people test their tanks much more religiously than they test their pre-tank water they are far more likely to look to the tank for the cause instead of pre-tank water problems.