It's a nice setup, bravo for getting such a lovely spacious tank! However as Scriven's has noted, it's highly likely these fish will out grow your current set up. Single tailed goldfish are notorious for growing very big! (As you've observed with super fish!).
They can be upgraded to a pond once they reach around 5 or 6 inches, so they have the room to grow to their full potential of 1 foot.
Quite often you can find local neighbours or parks with a pond who could take the fish and you could always take the children to see them in their new home
Or if you are able too, you could always have a pond yourself or consider a larger tank when the fish have grown large aquaria of 6 or 7 foot can be viable for 3 common goldfish
Just something to keep in mind
I think your tank would be a viable 'forever' home for some fancy goldfish though
Perhaps 2 or 3.
As for the ammonia, have you tested your tap water? I had issues with ammonia for ages and couldn't figure out what it was, until I realised there was a constant source coming from my taps!