Agressive Fantail Black Moor Goldfish
So Im a college student and have two Black Fantail Moor goldfish in a 10 gallon tank on my desk. Everything was going great (I've had both fish for several months) until I noticed Othello (the larger Moor, about 4-5 inches long) started to be agressive in short spurts toward my other, smaller (about 2-3 inches) Moor, Ella. He will buffet her with his fins when I put food in, chase her about the tank and push her around; even to a point of flipping her upside down and shoving her to the top of the tank.
Ive only seen it once or twice but it really concerns me; I'm at classes and activities for most of the day and while I return to my house regularly to drop off books or whatever, I dont see them every moment of every day. Most of the time they get along okay, swimming about and poking the bottom, but when it happens its very disturbing and I've been scared tot he point of intervention twice by rapping on the glass and even poking Othello gently when he was basically barrel-rolling poor Ella around the perimeter of the tank.
Is this a common behavior for goldfish, maybe Moors in particular? I've had fancy tailed goldfish in the past before Ella and Othello, and tetras, goldfish and gobies in the distant past, but I've never seen aggression like this. Are they just playing and I'm overreacting? Any thoughts on the matter would be great!
A Worried Goldfish Mom