Goldfish Illness

Fish is gasping on the surface of the tank to get oxygen
Swimming abnormally
Any swelling or lumps on its skin
Not feeding as usual
A thin body or strange bloating

Change the water partially

The main reason fish get ill is because of polluted water. Make sure that there is no excess food in the tank that is left to rot and always change the water partially on a regular basis.

Swimming to the top

If the water in the tank gets too hot the oxygen level in the water will quickly drop and the fish will be distressed, showing signs such as swimming to the top to get air.

Shade the fish tank

You should shade the fish tank from sunlight with a cloth or towel, put the filter on if it is not already or in extreme situations, float a bag of ice cubes on the top of the water to cool the water in the tank down quickly.