125 will just about fit one adult fancy goldfish actually. They grow to the size of a cricket ball/big orange , I've seen them myself. You would need 55 gals (200 litres approx ) for a pair so one will need re homing eventually.
I personally reckon that 2 fancys will be ok in 125l. I would say it's the bare minimum, but considering their eventual size and relatively inactive lifestyle (key work 'relatively' Ie. similar to bettas) it should be fine.
However...
It will need double filtration
Big water changes weekly (40%+)
Weekly tests, you need to check how quickly your nitrates are increasing inbetween water changes. If they are rising anything above 40ppm imbetween water changes then I do agree one will need re-homing. (though as they prefer to be in at least I pair then I would recommend both be re-homed and then you set up a community tank or similar.)