One of the most important decisions in starting a saltwater aquarium will be the size of the tank.
The basic rule of thumb is the bigger the better. A larger tank will be easier to control and gives a bit more leeway for mistakes (which are inevitable). The smallest tank for beginners should be no less than 20 gallons, with 55 gallons being even better. For someone versed in fish keeping (i.e., converting from fresh to saltwater), a 10 or 15 gallon tank will work, but is not suggested. In general, fish like long, wide tanks. The more surface area a tank has, the better the gas exchange will be and the happier the fish will be.
Before finalizing on a tank size, remember that fish densities are much lower for saltwater than freshwater. That is, you can not put as many fish in a saltwater tank as you can in a freshwater tank. Putting more than 2 saltwater fish in 10 gallon tank is asking for trouble. A general rule of thumb is 4″ (10cm) of small-to-medium fish per 10 gallons, or 2″ (5cm) of larger/fast growing fish per 10 gallons. This is just a rough estimate of the number of fish. There is no exact number since finding the stocking density has to take into account the filtration, maintenance, feeding schedule, etc..
Beyond the number of fish you wish to keep, the tank’s size will also affect your filtration and lighting choices, both in cost and design. Tanks which are 48 inches (122cm) long are usually cheaper to light because the lamps are more readily available. However, the larger the tank, the more light you will need to provide your inhabitants. Moreover, a larger tank needs efficient filtration to keep the system thriving. A good size tank is around 55 gallons.
As a note, scrutinize hoods carefully. Many of them are designed for 48″ tanks, but require two 24″ lamps rather than one 48″ lamp. (24″ lamps are usually more expensive than 48″ lamps.)
Avoid Direct Exposure to Sunlight
Once you have decided on a tank, make sure you have a place to put it. The tank should not be in direct sunlight or in an area which is very drafty. Also, make very certain the stand will be capable of holding the weight of the tank, plus substrate, plus rocks, plus water. In total, a 55 gallon tank will probably weigh over 800 pounds.