new goldfish owner

Question
Hello!
I recently bought a Black moor and a golden fantail, They seem to be doing fine, but just tonight when I came home I noticed the fantail seemingly bully the black moor....(these are my first fish so i do not know proper behavior patterns). I read that black moors should not be placed in a tank with highly active fish, im wondering if the fantail is too active.

I noticed that the golden fantail comes up from behind and looks like its motioning to bite the black moors tail... and the moor moves away but they don't look like they fight, I just dont know if the black moor is just to passive and I should not keep them in the same tank.


And one other question:

The black moor is the one that worries me because it seems pretty "chill" or calm and then it has a sudden wild movement every once in a while, I just dont know if thats normal.

thank you for your time

Answer
The size of a tank needed for two goldfish, by the time they are done growing is about 30-40 gallons. Goldfish typically can grow up to 7-8 inches and are incredibly (INCREDIBLY) messy fish. I don't tend to deal with them much because I don't like 2x a week water changes (or necessarily want to buy a 20 gallon tank for a full grown one).

Yes, this is bullying and you should probably put them in separate tanks or take the golden fantail back. Some fish just seem to like to bully others, much like middle/high school children (^.^).

As for the random darting, this could be caused by an outside parasite infection. It it seems to be scratching itself against rocks, plants, or the wall of the tank - something (ex. fish lice) could be affecting your fish.

Hope this helps!

-Bethany