I will let Robert Fenner answer with a informative reponse for those interested in real facts from a person that writes books and maintains a website related to all facets of the saltwater aquarium hobby.
"Frequent Partial Water Changes"
BobFenner@WetWebMedia.com
Probably the most important aspect of maintenance a keeper of an aquatic system can do to optimize water quality and health of their charges is to change some of the water is a regular basis. This Section deals with the reasons for, and some rules of thumb as to how often, how much and how to make these changes.
Rationale:
There are several major benefits of frequent partial water changes: Dilution of nutrient, removal of particulate matter, reduction in microbial populations and their metabolites. Results anticipated are faster, more vigorous growth, reduced algae growth, color and odor.
It has been written in many fisheries, limnological and hobbyist texts that along with temperature and photoperiod, metabolite ("wastes") build-up is one of the three most important factors influencing the health, growth and reproduction of livestock.
More specifically; in the trade, ammonia and other nitrogenous wastes are recognized as the number one killer of aquatic life in captive conditions. Not to say that all the "stuff" produced by the system's desiraable life is toxic. Some metabolites, like pheromones, are actually known to have calming effects. Therefore the concept of partial, not total water changes.
In doing these water changes we are interested in a dilution-solution; that is, keeping these so-called waste products at tolerable levels.
There are several ways this is otherwise accomplished. Most common are some forms of biological filtration and chemical filtration like carbon and ion-exchange materials. The last are useful but often labor and money intensive. Moreover, these chemical filtrants do remove desirable chemicals as well.
As stated in so many previous Sections it is imperative vital that as much extraneous materials: foods, dirt from decor, material from the immediate outside environment be kept from getting in the system. What little does make its way in should be removed by netting/vacuuming, diluted or removed by making partial water changes.
Some potential pollution will probably be added to your system in the way of food and chemical additives/fertilizer. Even without over- or mis- feeding and/or fertilizing, freshwater evaporation adds to a decided negative chemical effect on an aquatic system. This "Salton Sea Syndrome" occurs as water evaporates leaving behind its' chemical constituents.
So enough of the reasoning for making water changes; onto the nuts and bolts of how to do them:
How often:
Depending on your pump/filter/circulation system, stocking and feeding regimen et al., partial water changes about once a week to about once a month are about right. More frequent smaller amounts are better than infrequent mega-changes, with one possible exception. Some writers advocate an occasional massive change (50% or more) as a stop-gap measure to dilute metabolites, nitrates in particular. I'd rather encourage you to stick to regular, smaller volume changes; they're safer and accomplish about the same ends.
Make a schedule/notebook for your system and keep track of what you do and how it works for you. Patterns will emerge and give you a guideline for how frequent you should change water.
How much:
Five to ten percent for larger systems and twice that for smaller is generally sufficient. The chemical/physical/biological shock from changing too much too soon is to be avoided.
Though some marine authors state that water treatment chemicals are unwarranted with such frequent small percentage change, I'd encourage you "to be safe, rather than sorry", and treat to remove chlorine/chloramine unless you're preparing water a week or more in advance of use.
How to:
However it is achieved, the part of the water and what's dissolved in it are mainly to be found at and in the bottom.
Solids are systematically removed from part, but never all of the bottom of the tank and possibly sump by using a "gravel vacuum". We don't want to vent all the beneficial microbes along with the solids, so a plan is made to move the decor and vacuum a half, third, what have you, of the base in a given water change period.
New water is replaced with pre-mixed synthetic of similar temperature and specific gravity.
Summary:
Regardless of how well a system is designed and constructed, there will always be maintenance. Frequent partial water changes are one of the best ways of ensuring continuing success.
There are manufacturers who claim their products do away with the need for frequent partial water changes. Their products may well extend the amount of time between changing or ostensibly eliminate it, but at what economic cost?
With the proper tools and materials, water changes are a breeze. Water changing is the cheapest, easiest, most sure method of diluting wastes and replenishing buffering capacity, "trace materials", while concurrently cleaning the system of undesirable solids and liquids.
. As others have posted you can by DI water at walmart/etc for about .60 a gallon. Why do you need a water change so badly? I do know that people do it weekly. However, you could always wait a few weeks and do a larger change when the $$ allow. Heck if you need 10 gals or so Ill give it to you for free. You come get it