Advice already given is great as stated a larger tank or rehoming the fish
Until then, get the water tested and keep on top of water changes.
Goldfish need massive amounts of space and I imagine the previous fish could have died from poor water quality and over stocking a tank. It's an easy mistake to make (some of us have been there), so I understand it must have been hard to lose the fish.
I lost a common goldfish 2 years ago and have been caring for my one remaining common goldfish who I currently have in a 3 foot, 90 litre/23 gallons tank with a big external filter -hopefully this gives you an idea of what's needed to keep these little guys healthy and happy. I will be upgrading her to a pond when the time comes. Generally the bigger the tank the better, often you can get cheap tanks second hand
I hope that with water changes and improved water quality, the fishes eye will improve, I've never seen one have a red eye before.... Are there red areas anywhere else on the body?