Although it is important to choose an aquarium that you think will look good in your living room, there are other equally important considerations that affect the ongoing expense as well as the health of the inhabitants.

One of the first choices you will face is whether to purchase a glass or acrylic aquarium. The differences are listed in the box below.

In general we have found that stability of the environment is the most important aspect of successful fish keeping. A lot of material has been written about the ideal environment for various species of fish, whether they like warm water or cold water, high Ph or low Ph, hard water or soft water, but this is generally useful only if you are attempting to breed the fish or are planning on keeping a very sensitive fish that is known to be challenging.

In choosing all of the items and the fish themselves, always keep in mind the stability of the environment.

Size of The Aquarium:
Always choose the largest size that fits your living room and your budget. In a marine aquarium this is critical since the fish capacity is three to five times less than a freshwater aquarium.

The amount of oxygen available is determined mainly by the amount of surface area the tank has since oxygen enters the water primarily at the surface. Although aeration, with an air stone or a power head, seems like it is directly adding oxygen to the water, most of the benefit comes from moving water from the bottom of the aquarium to the surface where oxygen is exchanged for carbon dioxide.

Water movement at the surface also increases oxygenation and will generally increase the stocking capacity as well as the health of the fish. Keep in mind that air-stones and power-heads can (and generally will) fail at some time or another so don't overstock an aquarium to the point that your fish will die if the power goes out for a couple of hours! It is almost always more desirable to have a few less fish that are colorful and healthy than to push the limits of the tank and risk loosing some of your fish. Not only are fish happier, but the maintenance is easier!

In general, although we have seen successful marine tanks that are smaller, a 20 gallon is smallest size that we can recommend with 60 to 100 gallon tanks being much more desirable. The larger the aquarium, the greater your chance of having a successful experience.

Filters and Filtration:
There are three main types of filtration for marine aquaria. Several other methods that are more commonly used in reef systems can also be beneficial for fish only systems and will also be mentioned here.

This is the most important type of filtration for stability and reduction of toxic wastes. In any biological environment there will be a production of ammonia from normal respiration (fish release ammonia from their gills) and the breakdown of wastes. Ammonia is toxic to fish and in nature is reduced to nitrite by a bacteria called Nitrosonomas. Nitrite is also poisonous to fish and is further broken down into nitrate by bacteria called Nitrobacter. Nitrate is relatively harmless to fish but is a primary food for plants and algae. Frequent water changes (say 10% every week or two) will control nitrate levels.

Collectively the process of turning ammonia to nitrate is called nitrification and is carried out in the presence of oxygen (aerobic conditions). Denitrification is the process where nitrates are converted to nitrogen gas in the absence of oxygen (anaerobic conditions). Denitrification is generally not of concern to the marine aquarist unless the control of nitrates is desired (See Reef Systems).

Under-gravel filters: Under-gravel filters are the most common type of biological filter. They work by circulating water through the gravel. The bacteria grow in the gravel.

Trickle-filters: In order to increase the effectiveness and number of bacteria growing in the filter, the media, usually bio-balls or other medium with a large surface area, is brought out into the air and water is circulated over it. Since the bacteria are aerobic, they thrive in the higher oxygen content of air.

Live Rock and Live Sand: These are used in reef and marine systems as a biological filter. Live rock gives the added bonus of having de-nitrifying bacteria deep inside the rock to help remove nitrates.

Mechanical Filtration:
Mechanical filtration refers to the removal of particulate matter from the aquarium. Filter types include:

Power filters - forces the water through some kind of floss.

Canisters - with pleated cartridges act as mechanical filters. It should be mentioned that diatomaceous earth filters, commonly available in canister filters, can remove particles as small as 3 microns in size which really 'polishes' the water. They can be used to remove bacteria and algae blooms but generally clog after a short period of time and should not be used on a continuous basis.

Under-gravel filters - also act as mechanical filters by trapping debris in the gravel. Mechanical filtration is good for keeping the water clear and free of debris.

Chemical Filtration:
Chemical filters are used to remove things that are dissolved in the water, and therefore cannot be removed by mechanical filters. They are most useful in getting rid of the yellow color that often develops in aquarium water over time.

Activated carbon - is the most common chemical filter. It is used mostly in canister and power filters, it traps many substances in minute pores.

According to Martin Moe in his book, "The Marine Aquarium Handbook", some of the substances removed by activated carbon are: copper, ozone, chlorine, antibiotics, some dissolved proteins and carbohydrates, iodine, mercury, cobalt, iron, methylene blue (a common medication), malachite green (another medication), sulfa drugs, organic dyes, and many other elements and compounds.

**Note: Many of the common aquarium medications including antibiotics are removed by activated carbon and therefore should not be used when treating the aquarium for disease.

Zeolites - Recently many other types of chemical filter materials, called zeolites, have been developed to remove specific compounds such as nitrate and phosphate. Generally a fish only marine aquarium has no need of these types of filtration but can be used in a mini-reef setup.

Other filtration:
Foam Fractionators or Protein Skimmers - work by passing a large stream of small bubbles through a column of aquarium water. Many different substances (proteins) will adhere to the surface of the bubbles and be removed from the water in the foam that bubbles from the top.

Skimmers are beneficial because they remove substances before they enter into the nitrification cycle which reduces nitrates (at the source) and increases oxygenation of the aquarium water.

One of the drawbacks to skimmers is that they remove trace elements and iodine which, if needed by any of the aquarium inhabitants, must be replaced periodically.

Ultraviolet sterilization - works by passing water through ultraviolet light. If the water is exposed long enough, at a high enough concentration of light, bacteria, algae, and parasites can be destroyed by the light.

This is a somewhat controversial filtration method that some people swear by and others dismiss as ineffective.

Ozone - can be passed through aquarium water to oxidize all forms of organic pollutants and kill many of the same organisms that ultraviolet radiation does. Commonly used in skimmers, one should make certain the materials used are "ozone safe" and that the water is passed through activated carbon before returning it to the aquarium because any residual ozone in the water is extremely toxic to the fish.

According do Julian Spring and J. Charles Delbeek in their book "The Reef Aquarium, Volume Two", ozone is useful mainly in large public aquariums to keep the water from coloring. They also state that aquariums that use ozone generally maintain a slightly higher nitrate level.