Based on the tone of your post, I expect that you won't like the answers you'll get. IMO, a common goldfish can easily reach 10 or 11 inches and they are NOT slim-bodied. They need a 4-foot tank like a 40 gallon long or a 55 gallon at a minimum. The 30 to 40 gallons per fish, and 10 to 20 gallons for each additional fish info is about right.

If you have a 125, you can probably add 4, maybe 5 fish to it. However, there are other variables to consider. What type of filtration will you have? How often do you plan to do water changes? How much water will be changed each time? Goldfish have big appetites and produce a lot of waste.

Have you considered Paradise fish instead? :smile2::smile2:

 

I believe there may be different max growth sizes out there for each type but the comets I kept were pond fish on size (three hands to pick up). The fancy tails were pick up in both hands and I've seen them have trouble turning in the smaller size 4ft tanks. Recently heard of fancy ones that get more single-hand size. Their tails can be significant all by themselves imo.

 

You have to treat it like another large fish that likes to swim.
Think of it like Cichla (peacock Bass)
would you keep a cichla in a 3ft tank?

6ft would be nice you could probably have like 3 adults.
the wider the better tbh, they wouldnt care about the depth.

Of course you could grow out many in a 6ft aquarium, but as fully grown adults you would be looking at about 3-4 fish and they are BIG fish.
the fancy goldfish also get huge. not so much long but big wide powerful bodied fish.
some of the fancy varieties like Ryukin, lionhead and ranchu are really cool and personable aquarium fish that look very impressive when grown out.

 

We have a public park with lagoon for ducks and the common gold fish.
Weekend's bring the public out to feed the fishes and the ducks.
Might be surprised at how large a common goldfish get's.
Some I have seen in the lagoon are 12 inches easy.
125 gal for four of these large fish sound's about right.
Please don't start with the.. "well they are small now, and I'll get a larger tank soon"

Is how our lagoon became stocked with them

P.S.

A five gallon bucket in the ground is still a five gal bucket.
Can put lipstick on a pig, and it's still a pig.
Just sayin.

 

You should check out kokos goldfish site-has lot of info on goldfish keeping including the single tail varieties. IMO 40g minimum for 1 fish and 4 foot tank-commons can get to 12" or more and don't underestimate how fast they can grow or how much poop they produce-get the bigger tank!
20g minimum for each additional single tail after the first so 60g minimum for 2, 80g for 3 etc. But its better to understock, even consider 40gs per a fish-lets you be more lenient with water changes and reduce risk of disease.
Just because they start out small they won't stay that way-you'll be thankful you got a larger tank in the long run to dilute their waste and give them room to swim.

 

Possible? Yes.

Should you do it? Probably no. A 20 Long (not a standard 20, aka 20H) with adequate filtration might work for 2 fancy goldfish (fantails, etc.) if you put in the effort for doing water changes. But for the comets and other slim-bodied goldfish, it would only work for a short while. And unless you already have the tank (seems like a 55 is the consensus for 2 goldfish), I wouldn't try it.

 

I've kept a lot of comets (common goldfish) over the years, though mostly outdoors. I wouldn't consider keeping a pair on anything smaller than a 180. A 125 is only 18" deep and that's just not much space. These things get massive. A 125 for one specimen would probably work with frequent water changes, but I can't really imagine having a 125 and just wanting a comet for it.