Goldfish can be divided into two groups, single-tailed and twin/double-tailed goldfish.
A single-tailed goldfish is a variety of goldfish with a single tail such as the Comet, Common or Shubunkin (poor man's koi). These varieties are much closer to the wild form of the fish and as such are hardy but require more space.
Twin-tailed goldfish are often referred to as fancy goldfish. Fancy goldfish varieties are more delicate and have a 'fat' appearance. Some twin-tailed varieties are the Fantail, Pearlscale, Ryunkin, Blackmoor and the Celestial-eye.
It is not advisable to mix twin-tailed and single-tailed goldfish together, as the single-tailed goldfish are more active and swift and may bully twin-tailed varieties as well as forcing them to compete for food.
The biggest goldfish that are on record are primarily single tailed fish due to the fact that they grow quite long rather than in girth compared to a fancy goldfish such as a fantail.
Because a comet is a single tailed goldfish, and can grow very long, it will need at least 30 gallons on it's own
You may want to add a liner to be on the safe side, but you could have 2 fancy goldfish, one one common or single tailed goldfish. (goldfish grow big and fast)
double-tailed Celestial eye goldfish or Choten gan is a double-tailed breed of fancy goldfish that has a breed-defining pair of telescope eyes which are turned upwards, pupils gazing skyward.
Godwits likes eating goldfish and selfish
Yes they can. I raised them together many times.
no
Neon tetras, guppies, and bettas are warm water fish(they need a heater) and calico ryukins and shubunkins are cold water(don't need a heater) so none would really fit together except the warm waters. I have a shubunkin goldfish, common goldfish, and 2 dojo/weather loaches in a 29gal, but they used to be in a 10gal. I recommend another double tailed goldfish if you have the ryukin(single tailed will be faster and nip at the others fins and eat all of the food).
There is no rule against it but by cross breeding different 'types' of fish you are producing fish that are potentially worth less than the possibly pure bred 3 tailed parents. Purists would say you would be doing the wrong thing. I personally would say the opposite.
No. A 10 gallon tank is barely big enough to keep a single goldfish in.
Athletic and single.
The number of goldfish that can live together in a single tank depends on the size of the tank and the type of goldfish. In general, it is recommended to have 20 gallons of water per goldfish to ensure they have enough space to swim and thrive. It is best to research the specific needs of the type of goldfish you have and consult with a pet store or veterinarian for guidance on the appropriate number for your tank.