Hi. I would be so grateful if anyone could help with the following. I am not the greatest fish expert as I inherited my first goldfish and didnt buy it. So any help would be great.

I have a 125 litre fishtank which had two goldfish in it for a year. Three weeks ago one of them died. Five days ago I added another goldfish to the tank but today when we fed them it started to get very aggressive and chase the old goldfish around the tank. It has stopped now that the food has gone but Im very worried that i did the wrong thing by buying another goldfish. Can anybody help?

Thank you