Agressive goldfish

I recently bought a 10 G aquarium and after due cycling added 2
goldfishes (fan tails) about inch and half each. I added the 2nd one
after a gap of almost after 4-5 days. All aspects like NH3, odour, 24X7
external filter are fine & the fishes seems to enjoy it. But i have
observed for last 4-5 days ..one goldie (slightly larger in size) has
become agressive and keeps chaising the other one. he kind of tries to
bit the tail of the smaller one but i have not seen any damage caused
after carefully observing the tail of my smaller goldie.

Now they are fed well and kept well ,the aquarium has some decent
beautification (all artificial). The question is why this agression.
Are they close to mating age OR am I missing something.

Thanks
Kedar