Size. Goldfish start off small, but grow to be quite large -- even a foot long -- if you take good care of them. Many first-time keepers buy a small tank or bowl to house their goldfish, only to discover that they need to keep buying ever-larger replacement tanks. It's far more economical for you to buy a large enough tank at the outset. A general rule is to provide a 20- to 30-gallon tank for your pet fish, and add at least 10 gallons to that volume for each additional goldfish you might add. They grow large, excrete a lot of waste and need room to swim in order to be happy, which should come as no surprise!
The importance of a hood. Goldfish have been known to jump out of uncovered fish tanks. Buying a hood for your tank is an easy way to avoid this calamity, while also providing a platform for lighting.
Light. Opt for fluorescent lighting rather than the incandescent variety. You'll save on electricity and your light will give off less heat. Keep the lighting on for about 10 hours every day.
Gravel floor. Gravel is the best choice as the floor of your goldfish tanks for several reasons. First of all, it won't affect the pH of the water, as other substances like coral can (more on pH below). Secondly, gravel provides an excellent surface on which good bacteria can grow - bacteria that work to break down the harmful waste elements produced within your tank.