New goldfish

[Endorite]

My sister was begging me to add her goldfish to my tank and I finally agreed yesterday, but only because my tank was bigger, and we got them at the same exact place so I assumed they would be fine together (they were also approximately the same size, and I couldn’t see any reason to Not put them together). However, when I added her fish, I noticed one of mine was chasing it around, staying underneath it and by its belly. This hasn’t happened frequently, but I noticed it when I put them together, and I noticed it at feeding time this morning. I’m still pretty new to fishkeeping, and although I think I’m making really good progress with my tank, I can say with absolute certainty that I don’t think I’m ready for mating goldfish and the extra challenges that are sure to follow... do I have to separate them again? Or will they be fine together?

[shakaho]

They should be fine.  The chasing usually indicates eagerness to make a new friend.

[Robert Didomenico]

One of the most common cause of aggression and overcrowding. Ensure that you don't have too many goldfish in your tank. If you find that too many fish in your tank then you should find them for a new home. However, if your tank is large then you goldfish will fight each other if they are competing for food.

[shakaho]

I've never seen goldfish fight, and I've had hundreds of goldfish.  They will make a point of being the first one to the food.  Sometimes a big fish will grab a large pellet away from a little one who couldn't swallow it.