Whats The Disease Of Having A Gold Eye Why Did My Large Two Year Old Angelfish Die?

Why did my large two year old angelfish die? - whats the disease of having a gold eye

I just started a new tank of 55 gallons per week and a half and I ride in the first place tetras 6, then has four large angelfish in another container. I saw some red parrot and a gold has severum and when I got home, I saw one of my angels around two years (deceased). I wonder whether it might be a disease, or adding new fish, or maybe my tank is too small, or perhaps the fact that there is a new tank. I checked all water parameters and everything seems good. Ideas? I also noticed that some of my fish has what looks like the eyes reddened. In general, one of the eyes is red. Is this normal or a sign of illness? THANK YOU SO MUCH!