Goldfish tank size requirements are different for different types of fish. The fish we call single tails are goldfish like Comet, Common, and Shubunkin but the Jikin and Wakin should also be included in this group since they also grow very large.
These goldfish types more closely resemble the ancestor of the goldfish in size and body form.
They can grow up to about 1 foot in length and they’re very athletic. These types of fish, really need a large water volume. They’re typically much better off in ponds, but if they must be kept in an aquarium, you should allow about 40 gallons of space per fish.
So for example if you had 3 comets, they should be kept in a tank no smaller than 120 gallons. This ensures that they have ample swimming room and enough water for sufficient waste dilution.
The other category of goldfish are the fancy goldfish, or the double tails. There are many more varieties in this category including the Fantail, Oranda, Ranchu, Lionhead, Telescope eye, Bubble eye, and Veiltail.
These fish have a shorter and rounder body shape than the single tails, and while they can also grow very large, they don’t typically grow quite as large as the single tail types, and they’re certainly not as athletic.
Like the single tails though they are big waste producers, and still require a great deal of space. You should allow 20 gallons of space per fish. So for example, if you have 3 Orandas, they should be kept in a tank no smaller than 60 gallons.
With all types of goldfish, the more space the better. Many hobbyists tend to have healthier fish in general when they keep their tanks under stocked. An under stocked tank simply means that there are fewer fish in the tanks, and the stocking guidelines would allow for. an under stocked tank is actually much easier to care for as well.
Imagine you have 5 medium-sized fantails and a 55 gallon tank. You would have to do large water changes multiple times per week, to keep the parameters within acceptable levels.
Now imagine you only have 2 medium-sized fantails in that 55-gallon tank. While you will still have to do large water changes, you will have much more leeway.
If for some reason you missed a water change, it would not be nearly as detrimental as it would be if you had 5 fish in that tank.
An important thing to remember is that the tank is the fish’s entire world.
Look at it that way, and put yourself in that position, and design the tank that you would be happy living in if you were that fish. Would you want a lot of space to explore and move around? or would you want a little bit of space and cramped quarters?
I think you’d want a lot of space, and I think our fish do too.
I think everyone understands the temptation to get as many fish as possible in your tank because there are so many beautiful goldfish out there.
But keep in mind that less is more. By under-stocking your tank you will probably avoid many issues and your fish will be healthier and happier.