Goldfish (Carassium auratus) are often the first type of fish that people will buy as pets. They are very easy to care for and are graceful to watch; especially the varieties with the longer fins and tails. Originating in China where goldfish cultivation is still a very popular activity, the spectrum of novel goldfish types has expanded considerably as a result of intensive breeding programs. Over a hundred varieties provide the purchaser with a spectrum of choice ranging from the simple comet-type fish that you commonly find in the local pet store that might sell for a few dollars, to rare varieties that have elaborate coloring and fins that can cost many hundreds of dollars! These fish can be very long-lived if fed and "housed" appropriately. Taking time to set up a top-notch aquarium is a bit of an investment up-front, but will help the fish potentially enjoy a full lifespan, which can easily top 10 years!

Basic Biology and Aquarium (Environmental) Design

The goldfish is considered a very hardy species, and is tolerant of temperature changes, but still requires clean, chlorine-free water, proper water aeration and filtering, and enough space to move around. The little round goldfish bowls that often come packaged with a "goldfish-to-go" are not ideal aquariums. The typical round bowl is too small, especially for more than one fish, and does not provide easy setup of a filter or aerator system (a sale package would not typically include these latter components). A 10-20 gallon tank is not excessive for one or two fish, and the typical rectangular aquarium tank package provides a top that will mount a filter, and has entry points for an aerator system.

To ensure that the water is safe, make sure that it is de-chlorinated using drops available from the pet supplier, or allow water to sit for at least 24 hours (the latter is not considered as effective to clear chlorine). Take care to avoid temperature shock by checking the water temperature before you place the fish into the tank. When cleaning the tank, avoid strong chemical cleaners, and rinse everything very thoroughly since soap and bleach residues are toxic.

The tank should be placed out of direct sunlight because this may lead to overheated water, and sunlight favors algae growth. If the tank is set up correctly, very little water turnover is needed. Removing, and replacing 1/4 of the water volume every 7-10 day will suffice. Full tank cleaning should be done on an as-need basis, but in a good set-up, often full cleans can be done at 2, or even 3 month intervals.

Providing the fish with plants and toy caves is strongly recommended. They really enjoy the opportunity to "hide" in their cubbies and seem to feel more secure when plastic or real plants provide a bit of a hiding spot. Though a substrate is not necessary, the tank generally has some gravel or large glass beads as a base.
Always use a net to catch the fish, otherwise your hand may strip the surface of the scales of their valuable slippery coating, and it is much more stressful for fish to feel pressure of fingers (also easy to lose grip, and drop it!).

Feeding

The provision of flaked goldfish food is an adequate food source. The biggest problem with feeding is usually overfeeding―it can be lethal. Overfeeding will also result in murky water, with the net result that the tank needs to be cleaned much more frequently. Though fish can be fed twice daily, once daily feeding will also suffice. The fish will overeat if they are overfed! A pinch of food per fish per feeding is a good general rule. The "pinch" is an approximate measured dose that is equivalent to about 1/4 of an 1/8th tsp. Flakes should be crumbled into smaller pieces for smaller fish.
Hungry fish will come up to the tank wall when you approach. Sometimes they will put on a flashy swimming display, perhaps to attract attention. A healthy, hungry fish will immediately go after food that is placed into the tank. If normal feeding behaviour is not observed, the health care team should be consulted.
If they are left for more than a day, a weekend (or week) feeder shell should be placed in the tank if a pet sitter is not doing regular feedings. A pet sitter should be carefully shown how much to feed in your absence to avoid overfeeding.