Which type of goldfish are you thinking of combining? Some mix well, others don't. Ryukins shouldn't be with bubble eyes or other dorsalless fish and really shouldn't be with telescopes or orandas. Usual advice is to mix like with like -- single tails with single tails, egg shaped with egg shaped and dorsalless with dorsalless. I find most egg shaped goldies do fine together. Ryukins can safely be with singletailed goldfish and even hold their own with koi.
__________________ Tanks everywhere, a well-stocked garden pond outside and a Horde of Flatcoated Retrievers  |