What is water quality?
I’ll mention water movement first, because I tend to include this with water quality. Many do not, listing it as a separate item. I include it as efficient water movement ensures adequate gas exchange leading to sufficient dissolved oxygen, and the movement helps bring nutrients to and remove dirt from corals. Insufficient water movement brings problems in itself.
There are the standard water quality tests. These include ammonia (should be nil), nitrite (should be nil), nitrate (should be as low as possible, hopefully less than 10ppm), phosphate (should be nil) and pH (should ideally be between 8.2 and 8.4, but the first figure can be lowered to 8.0). SG is important (should be between 1.022 to 1.025 depending on livestock). There are elements that are checked for in reef systems, such as calcium and magnesium. Alkalinity is another.
All of the checks are for water quality, indicating if remedial action should be taken, by supplementary additions and the like. The aquarist by adhering to [tag-tec]aquarium maintenance[/tag-tec] can maintain generally high water quality, but this is not the whole story.
Not everything within the seawater is measured for. There is a long list of seawater constituents that range from high proportions to minute trace proportions. As far as I know, science cannot state that the X or Y constituent is not needed by fish and/or corals. They may well be needed as they are present all the time on the reefs. Manufacturers attempt to produce a salt that is as close to natural as possible.
A major problem with water quality would be the appearance of ammonia or nitrite, both being toxic, maybe because of a sudden extra load on bio-filters or too early stocking of a system.
Another problem that is often met is an excess of nitrate and phosphate, perhaps caused by a common mistake, overfeeding.
So we have the water change. Obviously, doing a water change will not have an effect on water movement. What if ammonia and/or nitrite is detected and livestock is stressed. If the bio-filter cannot cope for a while, then water changes will control the problem, the size of the change dependant on the level detected. Similarly, if nitrate and/or phosphate is detected at high levels, then reducing the level to a tolerable one is most easily achieved by a water change, again size dependant on problem.
The normal water change is the routine one, done as a part of ongoing aquarium maintenance. The routine water change is usually 10% of the system net gallonage weekly. This amount can be flexed a little with care after the first few months of an aquariums life, provided a close watch is kept on water parameters and livestock health. Routine water changes should continue though. Why? The routine water change ‘freshens’ the seawater. It puts back, or partly puts back, many trace elements that may be depleted by livestock. Protein skimming removes trace elements, and so does the use of activated carbon. It helps with a stable alkalinity level, which means there should be more resistance to the seawater moving toward a lower pH reading because of the pressures of acidity. A water change helps with the control of nitrate and phosphate, both nutrients of nuisance algae, so it follows that the threat of nuisance algae should be reduced.
Quite apart from emergency action, the routine water change is a really valuable tool in the maintenance of high quality seawater. This is a routine action that can be flexed with care, but should never be abandoned.