Some ways you can tell that your goldfish is sick are that it might start swimming slowly, upside down, sideways and around in circles. Its colour might get patchy or blotchy and its eyes might look cloudy and close a little bit. It might also go to a corner of the tank and just sit there for long periods of time.
No, unless the fish has some nasty parasite. It is just a little fish, which is mostly protein, water, and trace minerals. Though, you could get sick if you have an allergic reaction to the goldfish, or anything that happens to be in or on it.
It is probably sick
My best suggestion is to go to a fish store and supply them with the information about your sick goldfish. They might pinpoint the sickness and offer you some medication for your fish.
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
Lots of signs some are vomiting, diahera, not eating, not moving around as much.
the signs of being sick are having a headake or feeling queezy
Sometimes maybe when its sick or something.
because they are sick dummy
If your goldfish eat a lot it means it is lazy.if it eat a little that means it is sick
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.
He/she will act differently-maybe less active. Also you can see some signs like cuts/sores, white/cloudy eyes, fungus on the scales/body, & anything that looks out of the ordinary. My molly fish had cloud eye and I used melafix to treat it.