There are a few things hobbyists should consider when choosing an aquarium. The tank's dimensions, the species of fish, the species of plants, and the type of filtration.
The surface area is the most important factor when choosing an aquarium. The majority of the gas exchange in a system happens at the surface. An aquarium that is 36 inches in length by 12 inches in width by 18 inches in height (90cm x 30cm x 45cm) has the same amount of gas exchange as one 36 L x 12 W x 36 H (90cm x 30cm x 90cm).
Fish that are territorial set up their territory based on the area at the bottom or surface of the aquarium. Aquariums that are long and wide are the best for heavy stocking. Octagons, hexagons, and short/tall aquariums severely limit the number of species of fish you can keep.
Deep aquariums over 24 inches (60 cm) are more difficult to clean, plant, and decorate. Powerful glass cleaning magnets are usually used on deep aquariums. Deep aquariums should be considered when keeping large or tall plants.
Wide aquariums (front to back) are desirable for planted tanks. Wide tanks provide depth-of-field to the aquarium. Wide tanks allow the planting of foreground, middle ground, and background plants and decor.
All aquariums should be level on the bottom and be completely in contact with base support. Aquariums that are not level and equally supported across the bottom can break, sometimes after they have been set up for more than a year. Shimming the bottom may be necessary. A ¼ inch (.6 cm) sheet of styrofoam can also be used to distribute the weight of an uneven surface.
Glass aquariums have some advantages and disadvantages. Glass aquariums are often less than half the price of an acrylic aquarium. Glass aquariums have some disadvantages as well, they are heavy, most have a green tint, and are not as easy to drill holes for using sump style filtration systems. Many larger glass aquariums use tempered glass on the bottom. Tempered glass cannot be drilled.
Acrylic aquariums have some advantages and disadvantages too. Acrylic aquariums are light, clearer than glass, and easy to drill holes to add an external sump-style filtration system. You can easily install an internal overflow pre-filter for a sump-style filtration system or buy a tank with one already built in. Acrylic aquariums have some disadvantages as well, they scratch easily, they can bow, and they are typically more expensive than glass. Acrylic aquariums can have the back plate colored acrylic (black or blue). For tanks that will be placed up against a wall, a colored back is desirable.
The recommendation for choosing a tank is based mostly on the size. If the aquarium you plan to buy is over 100 gallons (380 L), you should consider acrylic because its weight makes it easy to move. A 125-gallon (475 L) glass tank is difficult to move for two men. On the other hand, a 125-gallon (475 L) acrylic tank can be moved by one person.