A salt water fish tank is an aquarium that mimics an ocean environment to support salt water fish verses freshwater fish.

Salt water fish tanks are considered more difficult to keep than freshwater tanks because salt water fish have less tolerance for fluctuations that can occur quickly within the closed environment of an aquarium.

Aquariums, when setup properly, operate as self-supporting ecosystems. In an ecosystem, artificially created or natural, every element relies on every other element for its stability. The art of fishkeeping becomes a balancing act of monitoring and maintaining specific parameters in order to keep the entire system stable. This is especially critical in a salt water fish tank because the ocean is an surprisingly stable environment. Life forms that evolved in the ocean do not have the physical ability to adjust quickly to radically changing conditions. Any changes that occur in the ocean occur slowly. For example, a quick drop or rise in the temperature of a salt water tank can cause salt water fish to take ill, even if the fluctuation is relatively small.

Aside from temperature there are many other factors to consider in a salt water fish tank. When the ecosystem is functioning properly, they should all take care of themselves, but they must be monitored. A typical reading for some of these parameters might be:

pH: 8.2
Nitrates: < 20ppm for salt water fish and < 5ppm for invertebrates
Salinity/specific gravity measured at 77F (25C) with a corrected
hydrometer: 1.022
Temperature: 77F (25C)
Alkalinity: 3.0
Calcium for reef keepers and invertebrates: 400-450ppm Ca++

Though keeping a salt water fish tank may sound daunting, it is a rewarding experience to recreate the ocean environment in your home. If new to fishkeeping, starting out with a freshwater tank might be a good idea. Freshwater fish come from smaller bodies of water and therefore can tolerate mistakes slightly better than their salt water relatives. Once you have the experience of basic fishkeeping under your belt, moving to a salt water fish tank is a natural next step. Note that freshwater and salt water fish cannot be kept together. Freshwater fish will die in salt water, and salt water fish will die in freshwater.

If considering a salt water fish tank bear in mind that the bigger the tank, the easier it is to keep the ecosystem stable. If a heater should fail, for example, it will take longer for the temperature to begin to drop in a larger tank and it will drop slower, relatively speaking, than in a smaller tank. This principle is true of every water parameter. The larger the tank, the more forgiving.

Another good reason to consider tanks of greater capacity is that salt water tanks support 50% fewer fish than their freshwater counterparts. A 30 gallon (114 liter) salt water tank is fully stocked with just a few small fish.

For those who take the plunge into one of the most popular hobbies in the world, you'll find many resources to help you along the way. With a desire to invest the necessary interest and time, you are bound to reap an experience rich with reward that gives much more than it takes. Nothing is more primordial to humankind than the ocean, and keeping a bit of it near is a magical gift that is well worth the investment.