Just want to say I agree with jlk here.
20 gallon tank, minimum, with double filtration, is recommended for fancy goldfish. This is a minimum. Some Black Moors get bigger than others, and 20 gallons can get tight for the larger specimens.
If upgrading to a 20-30 gallon tank isn't feasible, I'm sure the community can give you some stocking suggestions that will be much more appropriate for a 10 gallon if you ask.
Best!