There is no doubt goldfish, Carassius auratus is one of the most adaptable fish species. While the parent species being either/or the Prussian carp, C. gibelio or the Crucian carp, C. carassius are proven without a doubt to adapt to a range of climates the goldfish on the other hand is treated as the sensitive child. Well, not all goldfish but this is very much a cherry picking of different varieties thrown into hardy or not.

First lets split the goldfish up:

Single tails

• Common/hibuna: This is the typical goldfish with the short single caudal fin.
• Comet: Displays a more elongate caudal fin that might be more ribbon shaped.
• Shubunkin: This is a variety that is split into multiple different subvarieties depending on caudal fin type. The London shubunkin has a shorter caudal fin, the common body and tail shape. The Japanese shubunkin has that comet caudal fin shape. The Bristol shubunkin being unique with a large elongate but heart shaped caudal fin that holds it’s height and shape. The difference between the shubunkin and the first two is the calico patterning, black, red, orange and blue.
• Tamasaba and sabao, shorter ryukin shaped goldfish with white and red patterning originally bred by Japanese koi breeders. The Tamasaba has the longer caudal fin.
• Nymph, not so much a variety but a undesirable mutation of the double tailed varieties where a number of single tailed individuals are produced.

Double tailed, this is produced by a duplication event (Abe et al., 2014).

• Wakin: Very similar to the common goldfish but with a double tail. Not to be confused with mutations in commons, comets and shubunkin resulting in a double tail, I have seen full to partial splits at least 3 times, the wakin seems to have more of an arch to the spine at the back.
• Jikin: Similar to the wakin but deeper bodied, seems to lack that arching and with more of a flower shaped caudal fin. It is strictly red and white, ideally with 13 points of red but this is produced by certain methods.
• Fantail: The most famous and varies on quality or how much it matches the standards. This variety by standard should be deeper bodied but has a short heart shaped caudal fin.
• Oranda: There are multiple subvarieties from goosehead to lionhead oranda, this variety is a deep bodied fish with fatty growths covering the head or the top of the head. Caudal fin shape also varies depending on standard and variety from a straight long veiltail to a short heart shaped caudal fin.
• Ryukin: An extreme deep bodied and described by the GSGB (Goldfish Society of the British Isles) as having a bulldog like appearance, it has a hump behind the head resulting in it’s unusual appearance. Despite the name it is not originally Japanese as having originated from China arriving via the Ryukyu Islands, quite a few varieties often thought of as Japanese have Chinese origins.
• Lionhead: As of recent an uncommon variety, most sold under this name are in fact low quality oranda. It should have a large fatty head growth like the oranda but lack a dorsal fin and have a straighter back.
• Ranchu: There are multiple varieties of this, the original Japanese ranchu is known as the top view ranchu, uncommon outside certain exporters. Like most true Japanese fish to be judged from above. Lacks a dorsal fin with a nice smooth arched back, short caudal fin and an attractive fatty head growth. Side view ranchu are common in the trade.
• Tosakin: A rare variety in the UK, very similar to a fantail but has a long caudal fin that is spread out in a butterfly shape to be viewed from above. Reached close to extinction in Japan but is making a recovery.
• Izumo nankin: A rare variety, similar to the ranchu in it’s deeper body but lacks any head growth. Red and white patterning in a desired pattern.
• Veiltail: Uncommon outside the show scene, has a broad, lacking that forking long caudal fin otherwise similar to the fantail.
• Moor: A telescope eyed variety, it generally in the UK has a veiltailed caudal fin to be standard and must have more triangular eyes.
• Globe eye: Similar to the moor in being a telescope eye but the eyes are rounded, the tail can be short or long but generally rounded and forked. Demekins do somewhat fall under this but are almost like a cross between the ryukin and globe eye.

I didn’t list these varieties for no reason, body shape is extremely diverse between each and therefore the split only means a difference in whether there is two or one caudal fin. Fancy usually refers to any goldfish but the comet, shubunkin and common which if you get what I mean it really means very little as a term.

There is a long history of goldfish’s being kept and bred outside in not just Japan and China; but also Thailand, Java, the UK and the USA. It is not difficult to find this in societies alternatively in a greenhouse.

I myself have kept a variety of fancies outside year round but it requires serious thought.

What benefits could keeping goldfish outside have?

• The major obvious one is colouration, natural lighting perhaps it’s the UVB really enhances black pigmentation (melanin) in goldfish or preventing any loss. Any green water encourages red colouration.
• Potentially more natural food sources, many insects and invertebrates naturally become introduced to ponds.
• Natural seasonal cycles like they would have experienced as a wild species, it gives them a period of rest. It is obvious how seasonal these fishes are in the fact they spawn with seasonal temperature ques.
• The opportunity and ease of providing more space, ponds are generally cheaper then aquariums. This can allow for much more enrichment.

Considerations

• Goldfish still need water changes and filtration in a pond regardless of plants or setup.
• Only add fishes to ponds in the warmer months, May to September (based on UK temperatures), ideally 15-18c or above giving them plenty of time to adapt to temperature drops later on in the year and avoiding sudden frosts.
• Over winter and when temperatures drop the fishes will have a reduced appetite and reduced metabolism so feeding is best reduced slowly to a stop in winter to prevent any rotting of uneaten food. When temperatures increase again the fish can be fed again. Water changes aren’t required when the fishes have such a drop of metabolism.
• Goldfish are susceptible to predators so netting or grids above the pond is a must.
• While goldfish are more then capable of going anaerobic during periods of cold extreme cold resulting in a deep and thick ice layer should be considered in countries where this occurs. This might mean any fishes being taken inside between September to May. Depth of pond is also important as the bottom will create a refuge for the fishes, shallow is much better for body shape but the temperature will drop much more rapidly.
• Any fishes displaying swim bladder disorders should be removed and kept inside as the heat or cold can damage any floating fish.
• In summer a good filter should create enough aeration and goldfish can gulp but an air pump or fountain would be important to maintain oxygen levels.

You can see it’s not so simple but it doesn’t mean it’s bad to keep them outside. The importance of natural cycles is potentially very understudied and underrated within the aquarium hobby. Potentially it could even lead to a longer lifespan and healthier fishes but we don’t really know?

Both goldfish considered fancy and not are both farmed in hotter climates then ours in the UK so there isn’t so much logic behind their perceived variance sensitivity excluding the limited gene pool of some.

References

Abe, G., Lee, S. H., Chang, M., Liu, S. C., Tsai, H. Y., & Ota, K. G. (2014). The origin of the bifurcated axial skeletal system in the twin-tail goldfish. Nature communications, 5(1), 3360.