Freshwater:
This type of aquarium does not get its just reward. Many feel that freshwater aquariums are dull and boring; nothing could be further from the truth. Freshwater aquarium setups come in infinite styles with no two being the same, everyone has there own preferences. When it comes to freshwater there are basic themes which all tanks should be based on in order to be successful. First and by far the easiest and most common is Tropical. Tropical fish are the cheapest to buy and include the common names we all know, such as guppies, goldfish, mollies, swordtails, Etc. No special water treatment is necessary except to remove chlorine in some manner.
The next type and a bit higher on the demand meter is Amazon. This type of aquarium might be considered to be a biotope, only certain fish may be maintained in this manner. The water needs to be stripped of all carbonate hardness while maintaining general hardness and then the water must be slightly acidified by using peat moss, or bog wood to introduce tannins as a result making the water resemble tea. Water quality needs to be monitored and adjusted regularly. The types of fish which enjoy this environment to name a few are Tetras, Angels, Gouramis, Neons, South American Cichlids, Etc.
The third type of aquarium and the next level on the difficulty scale would be Planted. Planted aquariums require special lighting, CO2 injection and crushed slate substrate; additives should be introduced which contain iron to enhance lush growth. Tropical or Amazon fish may be kept in the planted aquarium as long as the water is adjusted to suit their needs and do not have a high veggie diet!
The fourth and I feel to be the most difficult are Discus. This fish has needs so specific it has a biotope all its own. The water must be filtered using RODI and stripped clean, softened with peat moss to make it very acid about 5.5 to 6.2 and temps kept about 85 deg F water should be changed Bi-weekly and mixed at least 24 hrs in advance.
Brackish:
Brackish aquariums are between saltwater and freshwater. The species of fish that can tolerate salt and those that require some are Scats, Shark Cats, Puffers, Some Eels, Mollies, and African Cichlids ( Although Africans tolerate sea salt well there natural environment is more volcanic salts and a high (alkaline)PH some as high as 8.9 ). This type of setup is easy to keep due to the lack of parasites and as long as the water is well aerated and filtered. Very few plants will tolerate this so tanks usually have elaborate rock work.
Saltwater:
Saltwater aquariums are just what the name implies. Only true Marine species can survive here, water must have the chlorine removed and a quality salt mix added in order to mimic the Ocean environment. Usually scarcely decorated with dead coral skeletons these tanks are prone to parasites and disease. Filtration is of the highest quality utilizing a Wet/Dry, Protein skimmer, and a UV sterilizer. Water should be changed at least once monthly and mixed 24 hours before hand. Types of fish include all Marine species.
Reef:
This is by far the hardest of all the aquariums. The use of RODI is a must quality salt must be added. Water changed no less than once per month. The use of additives is imperative to assure good coral growth. The frequency will vary depending on the quantity and species of corals kept. Equipment includes but is not limited to: Metal Halide or Compact Fluorescent, Protein Skimmer, Power Heads, Calcium Reactors, Kalkwasser Reactors, Etc. As you can see this should only be left to the professional aquarist even the best aquarium service company can not insure success with this type of setup as it needs constant care and attention to look its best.
FOWLR ( Fish only with live rock ):
This is a nice hybrid of the saltwater and reef systems. This combination usually results in a tank which requires less care than either system on its own. With low light and just a protein skimmer many novice aquarists have had great success with this method.
Paludariums:
A paludarium is an aquarium that is divided half water half land horizontally usually with lush hanging plant life, A vivarium is divided vertically and is more of a beach setup suited for fish such as mudskippers, or turtles. A paludarium is awesome for Archer Fish, and Four Eyed Fish. All types of fish and amphibians can be kept like this and it adds to the scope of the aquarium.