The goal of every aquarist is to maintain water quality as close to natural seawater as possible. In addition one must strive to keep the water parameters at consistent levels. This it true for any type of reef or fish only tank. Rapid changes in temperature, PH alkalinity, salinity or other water parameters will put an undo stress on organisms. Some are more sensitive than others but never less once an organism is stressed, it is more susceptible to disease and parasites. This may and often will spread to more hearty organisms. Below is a table of natural seawater parameters along with an acceptable range of aquarium water parameters.
Parameters Natural Seawater Recommended Aquarium levels
Temperature 72˚F-86˚F 74˚F-80˚F
Salinity 1.022-1.029 1.023-1.025
PH 8.0-8.3 7.8-8.4
Calcium 420ppm 380ppm-450ppm
Alkalinity 7dkh-8dkh 7dkh-11dkh
Magnesium 1280ppm 1250ppm-1350ppm
Strontium 8ppm 5ppm-15ppm
Boron 65ppm N/A
Iron 0.000006ppm N/A
Iodine 0.06ppm 0.03ppm-0.06ppm
Phosphate 0.005ppm <0.015ppm
Ammonia <0.1ppm <0.1ppm
Nitrite <0.0001ppm <0.2ppm
Nitrate <0.1ppm <0.2ppm
For more information click on the following links
Reef Aquarium Water Parameters
WWF Water Chemistry & Parameter Forum