Can anyone tell me if theyve experienced AGGRESSION in their goldfish tank

I recently placed a rhyukin, and a common goldfish in my (30gal) tank, with my fantail. At first they got along, however after a day or so, they started chasing my fantail around a little. Then my fantail became so timid, and avoidant, that he actually got sick. I have since removed him, and treated him. He's looking better. Perhaps he was already sick, and that's why they singled him out? Or are they just incompatible? I would like to put him back in the tank, but I don't want to stress him out again. Does anyone have any advice?

I'd like to second the person above me on their comment about "butting for eggs"

I once added a new fish to my goldfish tank and witnessed some horrific aggression. However, these were the same species and one turned out to be a male which was "bullying" - it was actually trying to get the other one, which was female to release her eggs. Once she did everything settled down. I found out that adding a female to warmer water then what she is used to can cause her to spawn when faced with a rampant male.

However, I am not sure if this is the case for you. You have mixed breeds and I think that your goldfish is simply being aggresive. They can swim much faster than a fancy and will eat all the food before your fantail can get any. Stress can cause a fish to become sick, and this is when they are most at risk of developing a disease.

I would seperate your fancy fantail from your common goldfish and rhyukin and keep him in a seperate tank or donate him to a friend who has a similar breed. But your fancy can live quite happily on his own, he does not need a tank mate and will probably be glad of the space!