Black Moor Needs Help

Hi all,

I'm new to the goldfish hobby. Bought a 10 gallon starter aquarium 3 months
ago with one black moor and two other small fish. All has been fine until
today.

The moor is moving very little, mostly floating belly up at the surface.
One eye has turned cloudy and the body has some red/orange discoloration.
Have been feeding flakes exclusively, but today the moor won't eat at all.
Water has always appeared a bit hazy but not too much. I tested today for
ammonia and results were good (almost none present).

The moor doesn't look like he'll make it through another day. Would
appreciate suggestions.

Thanks,
George