I've heard of this, but I always find some other reason for the "aggression". Since goldfish aren't a species that interacts much socially, they don't show what can be considered true aggression. Chasing and nipping can be because of sickly fish (I always quarentine for 2 months) or from breeding behavior. Goldfish will also swallow any fish small enough (they are opportunistic feeders). That's one reason you shouldn't put small sucker fish or algae eaters with them, they can die getting them stuck in their throat. A 10 gallon is too small to house a goldfish. I suggest just keeping your fish seperate because he eats other fish, not because he's aggressive.