Freshwater or Saltwater

Many aquarists are inspired to take up the hobby when they first see a well-kept marine tank (saltwater), with its exotic corals, swaying anemone, and flashy tropical fish. However, the costs and considerations when first setting up this type of tank can be intimidating to the beginner, and the freshwater tank, in general, is a simpler choice as a starter tank. There are a few major differences to consider when choosing between the two setups.


The most obvious difference between a freshwater and a marine tank is the addition of salt to the water. A freshwater tank needs only clean, chlorine-free water. Saltwater tanks require the addition of marine salt, which can be purchased at aquarium supply stores.

Unlike table salt (sodium chloride), marine salt contains a wide variety of soluble minerals that are essential for marine life. There are a variety of marine salts available, and your choice may depend on the type of species you would like to keep. A hydrometer is used to measure the amount of salt in the water as specific gravity. Once the aquarium contains the correct amount of salt, it can be topped up with fresh water unless you perform a water change; although the water itself will evaporate, the salt will not.


Most freshwater tanks require some form of filter – often a combination of air or water pumps and filtration substrate such as sponges, carbon, or gravel. In addition to this, marine tanks need a protein skimmer, also known as a foam fractionator. These devices use bubbles to froth up and remove contaminants from the water that are too small to be removed by conventional filters. They are needed to maintain the appropriate chemistry in the tank – marine species are more sensitive to nitrogenous waste than freshwater species.

Live rock is a form of filtration that has become popular even in the most basic, fish-only marine tanks. A chunk of long-dead coral serves as a home for a variety of algae and tiny organisms which help purify the water, process biological waste, and stabilize water chemistry. In addition, well-populated live rock will discourage the development of excess algae elsewhere in your tank.

Because of the requirements of live rock and other filter-feeding species, most marine tanks also need a powerhead to increase water movement. Water circulation is essential to filter-feeding species and will discourage stagnation; however, be careful not to overpower small or slow-moving species!


The ocean is much larger and more stable than the average lake or river, and as a result, marine species are far more sensitive than freshwater species to changes in their environment. Optimal temperature should be determined by species and monitored closely. Measures of water chemistry, such as pH, phosphate, nitrate, calcium, and others, should be performed frequently in a new tank, and occasionally in an established tank. Invertebrates such as snails, anemone, and coral are even less able to adapt to changes in water quality than fish species.

A larger tank will always provide a beginner aquarist with more leeway, as the larger volume of water will tend to absorb and curtail variations in temperature and chemistry. A 20 gallon tank is a good size for a beginner freshwater tank, but a 30 gallon or larger is preferable as a first saltwater aquarium.

In addition to the added expense of equipment and larger tank size, a marine aquarist will find that saltwater species tend to be more expensive than their freshwater alternatives. However, the cost and sensitivity of marine fish is an excellent reason to invest in quality equipment and an error-minimizing tank size. Keep in mind that marine fish also prefer lower densities than freshwater fish, so a marine tank will contain fewer fish than a freshwater tank of comparable size.

Brackish Aquariums

There is a third, lesser-known option in the saltwater versus freshwater debate: the brackish tank. A brackish tank simulates the environment found in swamps, rivers, and estuaries where the ocean meets fresh water, and there is a diluted salinity. Species from these environments (such as scats, monos, and gobies) tend to be more able to withstand variations in salinity and water quality than freshwater or saltwater species.

Brackish aquariums use basically the same equipment as freshwater tanks, with the addition of marine salt and a hydrometer to measure specific gravity. Not all aquarium supply stores carry brackish-water species, but some may be found kept in freshwater conditions, and slowly acclimatized to a brackish salinity.

By Jennifer Perret – writer