Water that is found in nature is rarely distilled or in pure form. It naturally contains dissolved salts, nutrients, and other elements dependent on where the water is found. The fish and coral that live in these waters have become accustomed to specific levels of these elements and therefore thrive in conditions that are suitable for them. Some fish or coral are hardier than other, in which they can survive in conditions that are less than ideal. But other fish and coral that are extremely sensitive to fluctuations end up dead in less than ideal conditions. This is why maintaining proper levels of (4) properties of water can increase the stability of your tank and encourage growth in a ideal environment. But why do these levels fluctuate in your aquarium? Hot days evaporate the water causing the water to become too salty, corals use up various compounds found in the water and these compounds need to be replenished, when inhabitants die they release toxins into the water that may affect the survival of other inhabitants, and more. To make sure that the water maintains proper water chemistry you should keep and eye on and test for these (4) parameters:
pH basically measures how acidic or basic the water is. If the pH is lower than 7.0, the water is acidic. If the pH is higher than 7.0 the water is basic. This is the reason why people say to drink milk when you have an upset stomach due to the low level of pH of milk that would try to balance out the acidic reaction from your stomach. Generally people try to keep their pH within the 8.0-8.3 range. Rapid changes to pH is hazardous to fish and coral, therefore sudden changes in pH is not recommended. When trying to raise or lower pH, try to do so by .3 increments a day.
Salinity is the measure of the total amount of dissolved salt in the water. Some people prefer to measure the specific gravity of the water in lieu of the salinity levels due to the close relation they have to each other. Generally salinity levels should be between 1.024-1.026. Or if you're measuring it by using specific gravity, that number should fall between 32-35. It is actually a bit difficult to have accurate salinity or specific gravity readings due to the not-so-accurate hydrometers that are sold in every fish store. In order to correct that problem, hobbyists use "refractometers" that measure salinity and specific gravity at a higher degree of accuracy. Due to this fact, these tend to be a bit more expensive (~$70) but will get the job done.
Calcium is vital to reef tanks that contain a lot of corals because hard corals require a steady supply of calcium and bicarbonate in other to build their calciferous backbone to grow. Calcium test kits are found in any general fish store and purchase of it should definitely be considered if you are planning to keep hard or stony corals in your aquarium.
Alkalinity is a complex concept in relation to corals but an important one. To put it simply, levels of alkalinity correspond to how resistant your water is to changes in pH. If the alkalinity levels are high in your tank, the water has higher capability to resist changes in pH, making it better for your inhabitants. Keep an eye on your dKH levels in your water, as dKH levels correspond with alkalinity. Again, tests for alkalinity are found in general fish stores everywhere.
Test your water often! Especially before and after you come back from vacation as leaving your water untouched for extended periods of time will most likely cause significant fluctuations in water parameters.