Know What's In Your Water
Aquarium water is to fish what our atmosphere is to us. In water, your fish and invertebrates breathe, eat, move about and shed their wastes, which is why a good filtration system and frequent water changes are so important in any aquarium - especially the marine tank.
If you think your water is just H2O, you're way off. Not only are trace elements present in any water - even distilled water - but when you mix in the salts for your marine system, you're changing the chemistry entirely. Where your water comes from and how to prepare it before you put it in your tank is a detail that's often overlooked by beginning hobbyists.
Many beginning aquarium keepers spend extraordinary amounts of money and frustrating hours of troubleshooting with their filter systems and tank accessories to try to figure out what's going wrong in their tank. More often than not, the problem is not the water; nevertheless, it could be, so know your water. Pre-treatment and testing are two simple efficient practices that will eliminate the possibility that you are adding a polluted atmosphere to your aquarium community.
Marine and freshwater hobbyists use various sources for their water. Many saltwater fish keepers don't have the luxury of living close enough to a high-quality saltwater source they can use for their home aquaria, so they too must begin with a fresh water source and then add salts specifically designed to create seawater.
Most freshwater sources include tap water, well water, rivers or lakes, distilled water, de-ionized water, spring water or rainwater. All of these water sources will have some level of undesired trace elements, although they are negligible in some cases, such as with de-ionized and distilled water. Unless you have a de-ionizer or distillation unit, purchasing this kind of water can get prohibitively expensive. But some of the other sources can be highly polluted such as tap water, well water and river or lake water.
Water Quality Tests
You should run water-quality tests to see what you have in your water before you put it in your tank. Some things you cannot test for; these include pesticides, organic phosphate, phenols and other compounds that in high concentrations can destroy your aquarium community. If you are going to use a water source regularly, such as your own tap water, it may be worthwhile to get it professionally analyzed to find out what exactly is in it and at what concentrations. Sometimes this can be the source of a chronic problem you have in your tank.
Regardless of the source, be sure to let your water sit for several hours before adding it to the tank - 24 hours is recommended. It is also highly suggested to aerate the water while it sits, which keeps oxygen levels high and can mean the difference between a healthy vibrant tank and one in which you are constantly battling parasitic infections and algae outbreaks.
For saltwater systems, add the salt before you let the water sit. Letting the salted water sit and aerate is most important for reef systems since the water in their natural environment is always highly oxygenated. Allowing the water to sit also permits common volatile gases that can harm your animals, such as chlorine, to evaporate. And, because some elements are less soluble than others and take longer to dissolve, the salt mixture will have time to dissolve, stabilize and distribute evenly in your container.
Most professional aquarists and many experienced hobbyists will pre-treat their "raw water" by letting it circulate through a small carbon and mechanical filter while it sits, which eliminates chlorine and small organic and inorganic particles and will neutralize many of the compounds that could cause problems once in your tank. This pre-treating will help reduce or eliminate fluctuations in source-water quality that you have no control over, such as chemical variations introduced by the water treatment facility that supplies your tap water or a micro-algae outbreak upriver from where you are getting your water.
Many brands of commercial salts are available and are usually of good quality, although they all vary slightly. For reef tanks, the quality of the salt mixture is of a greater concern since invertebrates such as corals and anemones are far more sensitive to tiny variations in the chemical composition of the water. Saltwater contains both major and minor elements, and the different brands fluctuate slightly from one another. (Never use generic salts or mixtures that don't list exactly what is in them.) Major elements found in all good quality salts include sodium chloride (i.e., table salt), magnesium chloride, magnesium sulfate, calcium sulfate, potassium sulfate, calcium carbonate and bromide salts. In seawater, these elements are always present in the same relative amounts regardless of where the water came from.
Differences among saltwater mixes that are just as important but a little more difficult to pin down are variations in the trace elements they contain, which can vary slightly from reef to reef even in nature. These trace elements make recreating "good" saltwater slightly challenging since some mixtures may create compounds, upon reacting with the water, that are harmful to your aquarium. Most high-quality saltwater mixtures have about 70 different trace elements that have proven to be sufficient to sustain a healthy aquarium. For reef systems especially, it is very important to make sure that the salt mixture doesn't add or promote the production of nitrates and phosphates. Most high-quality salts satisfy this requirement, but test the water before you add it to make sure.
Trace elements are important because they provide certain nutrients for your pets, which is why many professional aquarists maintain that you add a little extra of these compounds from time to time and not rely solely on water exchanges. These trace elements include iodine, strontium, molybdenum, iron, zinc and cobalt. For reef aquaria, in which the corals are actually growing, calcium may also be necessary. Commercial products are available that will fortify the water with these necessary trace elements. Deciding on whether you need them or how often you should add them is a matter of experience and careful experimentation.
Always test the water before you put it in your tank. This is most important for saltwater hobbyists since not all salt mixtures are alike. Good reef aquarium salts should be extremely low in both nitrates and phosphates, both of which could promote an algae problem and are generally toxic to your reef creatures. Recommended limits in "new" water are below 4.5 to 5 ppm of nitrates, and no more than 0.05 to 0.1 ppm of phosphates.