Thought I would throw out this article which is linked in Part two of this post by Carl Stohmeyer:
" It is also worth noting that most of these “Reasons for Water Changes” are inter related, so addressing one reason often has an affect on another and on the negative side of the equation ignoring one reason of has a domino affect on others
(1)* Nitrate control- A nitrate level of 15-20 ppm or less is best saltwater fish (1 ppm or less for Reef), 40-50 ppm or less for FW (you want a Nitrate level of at least 15 ppm for planted FW aquariums, as plants need some bio available nitrates in the water column). Use your nitrate level as a gauge (in an established aquarium) when to change your water. If your goal is to keep nitrates below 40 ppm, change water whenever nitrates reach this level.
I often have told my customers that Nitrate removal was an important reason (not the only) for water changes, and utilizing vacuuming as a method of removing organic mulm is an effective way of maintaining low nitrates (especially in the absence of nitrogen reducing anaerobic filter bed).
In freshwater aquariums (especially in the absence of a large amount of thriving plants), water changes using a gravel vacuum to remove ALL the organic mulm before it goes through the full nitrifying part of the nitrogen cycle is of the only sure way of controlling nitrates in a freshwater aquarium. The importance is amplified for tanks where Under Gravel Filters are employed (with these filters it is best to occasionally remove the lift tubes and siphon directly under the plate).
(2)*GH and Electrolytes ( positive mineral ions); this is related to the above point. Water changes will often replenish necessary electrolytes (when tap or well water are used). This is why RO water should ALWAYS be re-mineralized.
This is an important reason that is often missed in determining whether or not adequate methods, frequency, etc. of water changes are being utilized. If your aquarium GH (minerals/electrolytes) are falling, you are likely in need of a water change or should be employing products such as Wonder Shells and possible aragonite (although aragonite often will not respond rapidly enough to lower mineral levels as compared to Wonder shells, buffers or simple water changes).
Without adequate mineralization, your fish will suffer from poor osmoregulation and as well this will lower your Redox balance.
Importance of Calcium, magnesium and other electrolytes on aquariums(3)*Ph and KH control- Maintaining a proper ph (ph levels have a tendency to drop from biological activity), KH is the buffering ability of the water and that too can decrease. KH is VERY important and a major problem I have seen over the years in my maintenance business when calling on customers who seemed to always have problems with their aquariums and claimed cleaning their aquarium caused their problems, which of course is not true. Maintaining a proper KH between cleanings is important. SeaChem Buffers and to a lesser degrees aragonite or Wonder Shells may aid in this as well.
It should be noted that is large volumes of water changes are necessary for whatever reason, it is important to make sure that the new water added does not result in “roller coaster” pH reading, for which the osmotic stress on the fish is usually much harder than pH reading that is less desirable, but is usually tolerated fish more than many aquarists realize. This consideration is especially important with fish such as Discus where a water change may cause a sudden spike in pH (unless re-mineralized RO water is used that is also pre-adjusted to pH).
(4)*Lowering of DOC & Removal of Organic Mulm (Sludge); DOC stands for Dissolved Organic Compounds (some refer to the ‘C’ in this abbreviation as Carbon which would relate to carbon based molecules). This is inter-related to some of the other reasons such Redox, Bio Load, Nitrates, even KH & pH (has often high DOC will have an end result of lowering pH). This also includes removal of mulm/sludge from under under-gravel filter plates (placing a siphon tube directly under a plate often has worked well for me) as well as removal of this mulm/sludge from canister and sometimes even wet/dry filters.
This is one of the more important reasons from my years of experience as well as some controlled tests since DOC in the water column and Organic Mulm/Sludge under the gravel, decorations, rocks, etc. can have a profound effect on so many aspects of aquarium health such as the previously noted Nitrates, Redox, and even opportunistic diseases, in particular Saprolegnia/Fungus and Aeromonas.
This is achieved two ways in cleanings:
 Simply removing water on a regular basis removes DOC from the water column.
 The second aspect is surprisingly misunderstood, even by experienced aquarists and that is by thorough vacuuming of organic mulm, you remove a major cause/source of DOC. This is amplified more by the use of re-circulating micron filter cleaning methods such as the Aquarium Cleaning Machine where as you continue to vacuum the mulm and detritus even after you are finished removing water, after which you throw out the debris/mulm caught in the micron filter which would other wise add to the DOCs in the future. A Vortex Diatom Filter can perform this as well, although the gravel must be constantly stirred to make this device fully effective, which not only is this not usually possible, but this is also much more stressful to fish (I have used Diatom filters for years and they are still excellent devices, however when the Aquarium Cleaning Machine became available, it blew the Diatom filters out of the water).
Another device, the electric (battery powered) “Bag” vacuums do NOT compare to the Cleaning Machine or Diatom Filters. These use a bag that does not even trap 25% of the DOC causing mulm that a true micron filter can (such as the two noted above), my tests measuring Nitrates, KH, & Redox show this.
An exception to this would be the Eheim Sludge Remover Battery Vacuum, as it uses a much better reusable micron filter than the older style “Bag” battery Vacuums. Although the Eheim Battery Vacuum is not nearly as efficient as the Aquarium Cleaning Machine, it provides a reasonable alternative for small aquarium owner or those on a tight budget where the Aquarium Cleaning Machine is simply not practical. I should be noted that none of these devices still take the place of a water change in my experience/tests, however at least the Aquarium Cleaning Machine allows for discharge of water prior to switching to re-circulate.