Despite popular opinion, goldfish (Carassius auratus) are not "maintenance-free" or "trial" pets. They require and deserve just as much care as any other pet. Goldfish are hardy species and good at tolerating many beginner's mistakes, but you will need to put in some effort to ensure their health and survival. They should not be kept in a fish bowl, but require an aquarium with proper water aeration and filtration.
Types of Goldfish
There are many different varieties of goldfish. They are commonly divided into standard (long-body) and fancy (round body) varieties. The common Comet Goldfish is the most common pet fish species. Although they are often sold in the pets stores very small, these fish can grow up to 14 to 16 inches long and may require up to 100 gallons at their full size. Shubunkin and Sarasa goldfish varieties have the same body type, but are often more colorful fish, with combinations of red, white, black and yellow colors.
Fancy varieties of goldfish are numerous and have many anatomical adaptations. These breeds include the Oranda, Ranchu, Telescope, Fantail, Celestial, Moor, and other varieties. They have egg-shaped or rounded bodies. Given their body structure, they usually do not grow as large as long-body goldfish and may have some structural issues. These fish are more prone to buoyancy issues, spinal disease, and traumatic injury due to their protruding eyes and delicate fins. The Oranda goldfish has a fleshy growth on its head, called a wen, that may even need trimming if it overgrows the fish’s eyes or face.
No matter what variety of goldfish you keep, maintaining a proper aquatic environment is critical. You will need at least 20 gallons of water per goldfish, and expect your long-body goldfish to outgrow this within the first 5 to 10 years. Comet goldfish can easily live into their 20s and some have lived over 40 years, with the fancy varieties usually surviving into their mid-teens.
Most goldfish are very tolerant of beginner water quality mistakes. They are very hardy species and can tolerate moderate levels of ammonia, nitrite, and nitrate in circulating tanks. Once your aquarium nitrogen cycle has been established, it is recommended to keep your nitrate less than 20 mg/L, and the ammonia and nitrite at zero. In densely packed tanks, this may be hard to maintain without frequent water changes. Goldfish are notorious for constantly eating and creating a lot of waste—hence the spatial needs. They can also tolerate a wide range of pH, provided your kH (Carbonate Hardness = alkalinity) is high enough to stabilize the pH levels.
Goldfish are natural foragers and may spend most of their time digging around in the substrate of their aquarium or pond looking for food. They like to suck every last morsel out of their meals and will often pick up small rocks in their mouth to clean off food particles. These have a very low chance of getting stuck in their mouth, so don't worry about your fish picking them up.
Do not use small gel bio balls in goldfish tanks. These may get stuck and cause oral foreign bodies that can kill your fish.
Housing Goldfish Outdoors
In addition to indoor aquariums, goldfish can also be kept in outdoor ponds. Their hardiness makes it easy for them to stand up against temperature changes throughout the days and seasons. Provided your pond is large enough and properly built, your goldfish may enjoy spending all their time outside, or, in colder climates, splitting their time between outdoors and indoors. Outdoor ponds should have at least 30 gallons of water per goldfish and should have aeration and a filtration system. The water depth should be a minimum of 3-4 feet, deeper in colder climates.
It is not recommended to put fancy goldfish varieties in large, outdoor ponds. They are not well setup to swim large distances and the increased depth can put additional pressure on their swim bladder, augmenting buoyancy disorders. Due to their poorer swimming ability, they cannot compete with comet goldfish or koi for food, and they are more prone to predator attacks.
How Much and How Often to Feed Goldfish
Goldfish are often compared to Golden Retrievers for their similar appetites and ability to gorge themselves. They love to eat and, depending on the temperature of their water, may seem almost insatiable. No matter what fish species you keep, since they are ectotherms, the warmer the water, the hungrier your fish will be. With water temperatures below 70F (21C), feed your goldfish at least once a day. Above this, you will need to feed your goldfish twice a day. Give the fish as much food as they will eat in 3-5 minutes.
Look for a goldfish diet around 30-35% protein and 5-7% fat. For reproductively active, breeding fish or juveniles, you will need to select a diet higher in protein and fat. There are many commercial diets available within this range for goldfish. Weekly protein-rich treats, such as bloodworms or brine shrimp, are appropriate only in warmer water. Shelled green peas, not actually a cure for "constipation," are a better treat to feed more often.
Fancy goldfish and their buoyancy disorders can be related to diet. Being voracious surface eaters, a goldfish may suck in too much air during feeding and become temporarily positively buoyant. Switching to a sinking diet, or vice versa for negatively buoyant fish, will take advantage of their physostomous anatomy, where the gas bladder is connected to their digestive tract at the pharynx, and correct the buoyancy disorder without veterinary intervention.
Goldfish Maintenance Requirements
Although they are tolerant of beginner's mistakes, goldfish aquariums and ponds require regular maintenance. You will need to properly clean your filters on a weekly basis, but do not replace them all at once, as you may lose the necessary bacteria in them that break down fish wastes. Use a gravel siphon at least once a week to remove the debris and detritus from your substrate and top off your tank with conditioned fresh water.
As with all fish systems, it is highly recommended to purchase a liquid-based water quality test kit to maintain good water chemistry. Check it before and after your water changes to keep a close eye on the water quality in your goldfish aquarium.