Depending on what your sick goldfish has, it will either be specific to that fish, which you should place in a separate tank to treat, or it's something that will spread to other fish in your tank, in which case you will need to treat them all.
away from the sick any
Goldfish have a big red eyes because it could be do to its condition or because it could be getting sick also poor hygiene and no proper care so observe and keep it healthy
It is probably sick
Obviously they can get sick just like any other creature.
they might get the "ICK" or they can get sick from there bowl or tank not being clean
separate the sick fish from the othersbuy some aquarium salt ( this should help it heal)buy some fish medicine that you put into the water of the tankcheck your filtercheck your PH in the watergo to the pet shop for more info and help
Sometimes maybe when its sick or something.
because they are sick dummy
a goldfish lives with water to breath in if there overfeed they can still live they just need water to breath you should change the water evrry week if you cange it before then a week they get sick then they die
If your goldfish eat a lot it means it is lazy.if it eat a little that means it is sick
They should because they could get sick.
If they are little white bumps just on the gills, then you have a male goldfish that is ready to breed. If the white dots cover its body then your fish is sick with ick, and you will need to go to the petstore to get meds. for the fish.