Like most fish, goldfish are opportunistic feeders, meaning they will eat whenever food is available, whether they are hungry or not. This habit can be fatal. Their digestive tract often become so jammed with food that the intestines tear open, killing the fish. Also, an excess of food means more waste and feces, which will pollute the tank. Goldfish need to only be fed as much food as they can consume in three to four minutes, and no more than twice a day.
An effective method to determine if your goldfish is being properly fed is to look at their feces. They should be short and chunky, the same color as the food the fish is eating. Long strings of waste that trail behind the fish as they swim could be a sign of over-feeding.
Care has to be taken when choosing the right food for them, because goldfish need less protein (which they cannot digest in excess) and more carbohydrates. However, specialized food for them can be found on the market. Most come in the form of flakes, which float at the top of the aquarium or pellets, which sink slowly to the bottom.
Proper goldfish diet usually consists of a good quality floating pellet type food, along with occasional feedings of peas (removed from their outer skins), blanched green leafy vegetables, and bloodworms. Young goldfish benefit from the addition of brine shrimp to their diet. It is a good idea to set up a feeding ring in their tank where food is always introduced - not only does this prevent food from getting sucked into the filter, but it also ensures the fish know where to go for food. Within a week of its introduction the goldfish should be spending a lot of time at the feeding ring looking for food. Blanched greens should be clipped inside the tank where the goldfish can easily nibble at them.
Fancy goldfish such as Fantails and Black Moors benefit from having their food soaked first as feeding at the surface can increase their risk of getting swim bladder problems (where the fish is unable to control buoyancy).
It is a better idea to introduce blanched greens to the tank than it is to use live plants as a food source. Any plant that can grow fast enough to survive a tank full of voracious goldfish is likely to overrun the tank, creating a maintenance nightmare. On the other hand, tamer plants are likely to be uprooted, or simply torn to shreds, by the efforts of the goldfish.
The goldfish is quite hardy, which accounts in part for its popularity. Their supposed reputation in some areas for dying quickly is often due to poor care amongst uninformed buyers looking for a "cheap" pet. The goldfish is usually classified as a coldwater fish, as it can live in an unheated aquarium or in an outdoor water garden. In a pond, it will even survive brief periods of ice forming on the surface, so long as there is enough oxygen remaining in the water and the pond does not freeze solid.
Like most carp, goldfish produce a large amount of waste both in their feces and through their gills, releasing harmful chemicals into the water. This also happens because goldfish cannot digest an excess of proteins, unlike most tropical fish. Build-up of this waste to toxic levels can occur in a relatively short period of time, which is often the cause of a fish's sudden death. Although goldfish were historically displayed in small "goldfish bowls", a healthy and happy average-sized adult goldfish requires at least 10 US gallons (38 L) of water and above in order to live even a small life. In fact, for single-tailed varieties, such as commons or comets, it is really necessary to have 100 gallons (378 L) (for adult fish). Other goldfish experts say that it is the amount of water surface area, not the water volume, that decides how many goldfish may live in a container; one square foot of water surface area for every inch of goldfish length (370 cm²/cm). For example, if you had three goldfish of length 4 inches (10 cm) each, you might need 12 square feet (1,080 cm²) of water surface area. Surface area is an approximate measure of how much oxygen may be absorbed into the water from the air. If the water is being further aerated by way of water pump, filter or fountain, more goldfish may be kept in the container.
A quick rule of thumb to remember for common and comet goldfish is 40 gallons for the first and 20 gallons for each additional. For the fancy varieties it is 20 gallons for the first and 10 gallons for each additional.
A frequent misconception is that airstones do not increase the amount of dissolved oxygen in the water. Most of the transfer of oxygen occurs when the bubbles are formed at and around the airstone, and therefore airstones that create smaller bubbles in large amounts are preferred over those creating fewer larger bubbles. The ripples they create at the surface of the water also increase the surface area and therefore facilitate oxygen exchange at the surface of the tank. For the beginner acquiring a goldfish, the best advice that can be given is to get a 40 gallon long tank, these are typically inexpensive, have a huge surface area, and will be a suitable fish home for over 2 years. Beginner filtration can simply be provided by a box filter hung over the back of the tank, though filtration should be rated for ), as the ammonia levels produced by goldfish are much higher than those produced by the tropical fish filters are typically rated for. It is also imperative that adequate biological filtration is provided, as the breakdown of nitrates occurs much more slowly at the colder temperatures goldfish are kept at. One useful compromise is to keep the aquarium heated to a constant 72° F (22°C), as this temperature keeps the fish active while at the same time aiding bacterial growth.
In the end, it may be easier to transfer the goldfish to a pond rather than to a 100 gallon tank (378 L), as few homes can support the size. This is an area where most serious aquaculturists will agree, ponds are superior to tanks in the long run for goldfish.
Despite the small size of goldfish, they are able to grow large in a short amount of time. Therefore, a goldfish pond or a massive tank is often more of an investment than typically expected. Inexpensive ways to keep goldfish in more humane conditions until such an environment can be arranged by constructing an emergency home for them out of large plastic or glass containers, which can be as simple as 20 to 40 gallon (76-151 L) plastic boxes. These containers will house a fish much better than a small bowl, though proper hygiene must still be maintained. A 20% change of the water twice a week and using an aquarium vacuum to clean any dropping off of the bottom are effective methods in preserving water clarity. Additional rocks and plants in the tank help to remove nitrates from the water and also keep the fish entertained. A more ideal eventual solution is a 100 to 200 gallon long tank, and while this may seem excessive, common goldfish can grow to a foot (30.5 cm) long and may require such a large tank to have enough room to swim. Goldfish kept in smaller tanks will have much shorter lifespans.
Starting filtration should be provided by a box filter that hangs on the back of the tank. Undergravel filters have their uses, but they are inappropriate in a goldfish tank as they will quickly clog with detritus. Another excellent argument against undergravel filters is that the beneficial bacteria can sometimes die off, yet the filter will still appear to be working. The resulting spike in ammonia is often enough to kill every fish in the tank. Canister filters are another excellent choice, but usually well outside the budget of a beginning aquaculturist. Wet-dry and protein skimmer options are usually reserved for marine aquariums, so it would be extreme overkill to use them here. Simply put, the best option is to purchase a box filter rated for twice the volume of your tank, as goldfish produce copious amounts of waste due to their large size. An external box filter is preferable.
Lighting is the one area where someone on a budget can avoid investing money initially. Goldfish need day and night cycles to be healthy, but for tanks under 30 gallons (113 L) the room lights will be sufficient, at least temporarily. Another excellent area to save money is on the full hoods often sold at a greater cost than the tanks themselves at the pet store. A much more economical cover for the tank can be constructed out of the plastic mesh backing used for making loop-and-stitch rugs. This can be easily found at any craft store, and can be cut to fit the filters for your tank. Suitable covers can also be constructed out of particle board, which can be found at any hardware store. However, if a source of natural light is available (e.g. a window), the fish will be much happier than if they were under artificial lighting.
One last point that should be made for beginning goldfish owners is on the use of a heater. Goldfish may be coldwater fish, but this does not mean they can tolerate rapid changes in temperature. The sudden shift in temperature that comes at night, for example in an office building where a goldfish might be kept in a small office tank, could kill them, especially in winter. Therefore, even for cold water fish under certain circumstances, it is recommended that a tank heater be set to 68-72º F (20-22° C) and left in the tank year round. This is especially important for fancy varieties of goldfish, as they are less hardy than their brethren. Bear in mind, however, that temperatures over 25 degrees C can be extremely damaging for goldfish (this is the main reason why they shouldn't be kept in tropical tanks). As long as the room temperature does not fluctuate massively or drop extremely low, a heater can be left out of the aquarium.
As a footnote on tank sizes, a couple of very small goldfish will be fine in a 10-15 gallon (38-57 L) aquarium for a couple of months (with good filtration and water changes twice weekly). Beyond this time, however, the owner begins to run the risk of stunting their fish. A 40-50 gallon aquarium would suit 4 fish for a couple of years, however, in the end, a 100 gallon tank (or larger) needs to be obtained in order to suit the needs of the fishes.
There is a consumer stereotype that goldfish can be kept in small bowls. Unfortunately, the risk of stunting, deoxygenation, ammonia/nitrite poisoning caused by such a small environment means that this is hardly a suitable home for a gold, let alone any fish. It is truly funny and sad when somebody says that their fish 'lived for 4 years in a bowl and grew to a whopping 3 inches!' (7.5 cm) Most goldfish don't make it past a week in these unsuitable environments. Even among otherwise excellent aquarists, there is still an opinion that it is acceptable to keep these fish in small (under 50 gallons) tanks. Perhaps these people have never seen a full-sized goldfish, and this is not surprising, considering the fate that most golds suffer.
There exists a great deal of conflicting information on the best way to care for a goldfish, or rather, on the best feasible ways to care for one. The best solution is to build an indoor pond, as this would provide the benefits of an outdoor pond without the risk presented by raccoons, kingfishers, children, and weather. As for indoor care, there are a few general tips that should be noted.
- A quality mechanical filtration system rated for at least twice the volume of your tank is optimal for most tanks. Goldfish produce copious amounts of waste, and this factor must be constantly dealt with to prevent ammonia spikes.
- Undergravel filtration is usually not recommended for most goldfish tanks. These systems require up to six months to establish, and goldfish produce far too much waste. The filter plate will clog with detrious and become useless within a matter of weeks. Not only that, but these systems are only effective in tanks under 40 gallons, while most goldfish pairs or trios require 55 gallon tanks by the time they are fully grown.
- Floating pellets (particularly floating pond pellets) are one of the best staple foods for fish. Uneaten food can be removed easily, and the fish have no trouble finding the food before it dissolves.
- A twenty gallon long tank is usually a good choice for starting fishkeepers. While most will eventually have to upgrade to fifty gallon tanks, it is best for a beginner to have a tank that doesn't create additional problems. Larger tanks require more planning, more initial investment, and more mechanical inclination in setting it against the right walls and floor joists of the home. It is also nice in the long run to have the twenty gallon tank to use in case of emergency (such as a bigger tank breaking) or as a hospital/quarantine tank.
- An excellent idea when moving from a smaller (10-20 gallon) to a medium (37-55 gallon) tank is to use your old filter, and add an additional identical unit on the opposite end of the tank. It is nice to plan your filter so that when you move to a larger tank the filter can handle it as well. Two medium size mechanical filters working in conjunction will do wonders for your water quality, as well as ensure that if a filter were to break, there is at least some filtration until a replacement can be obtained.
- Air pumps are becoming outdated for most standard filtration setups. Air-driven mechanical filters are simply not powerful enough to deal with larger tanks, and should be reserved for fry tanks. The noise of most air pumps is also annoying. Most decorations driven by these air pumps do little to improve the oxygenation of the water. Another simple argument against the air pump is that it does not deal with the problem of tank overcrowding - if fish are gasping for air (lack of dissolved oxygen) it is almost always better to remove fish from the tank rather than resort to gimmicks to try and increase the air. You never want your fishes lives to depend on an electrical device being able to run constantly, as in the event of a failure their lives would be endangered.
- Lastly, keep the tank cold. Goldfish cannot be mixed with tropical fish, and indeed it is not even a good idea to mix them with other fish. Most catfish (i.e. plecos) that can be kept in coldwater get too big to keep in the tank, and they are a danger to the goldfish. Numerous accounts can be found of cases where a goldfish was injured, and a catfish will take a liking to sucking on the wounds. There are also a few stories of celestial goldfishes eyeballs being sucked out by a wandering algae eater. The bottom line, don't mix.