No, it isn't. A 10g is too small. Goldfish are sociable creatures therefore you cannot keep only one. A 40g is my recommended starting point if you want to try fancies. It can fit at least 2-3 goldfish. A lot of fancy goldfish are bulky growing at 8-10 inches. My oranda is 7" and that's still not enough for him. Aim for 10-15g per fancy goldfish but that doesn't mean anything smaller than a 30g is feasible because it is not.