They will have symptoms that are different from normal goldfish behavior.
Goldfish are fast swimmers.
If your goldfish is swimming very slowly and turning belly up frequently while swimming know that there is something wrong.
Goldfish sick with bacteria or fungal infection will have changed body color (usually pale color in comparison to their original color).
There may be different colored marks all over the body.
The stomach may become hard and fat.
There are some basic steps to prevent getting goldfish sick with disease.
The water of the aquarium should be frequently changed and treated to keep it clean.
There should be enough natural plants in the aquarium.
Certain goldfish diseases can be treated by adding aquarium salt to the water.
Variation in the water temperature that's not suitable for the goldfish may make them sick.
So sometimes the cause of goldfish sickness could be as simple as unclean or unsuitable water.
Aquarium or pond water should not be polluted with food stuff and goldfish poop.
The bottom of the aquarium should be also clean at all times.
Goldfish sick with disease should be separated from other goldfish in the aquarium.
They should be treated for the disease and taken back only when they are completely free from the disease and any form of bacterial or fungal infection.
You should know about all symptoms and causes of goldfish disease.
If you are ignorant, you may end up buying goldfish sick with disease.
Healthy goldfish in your aquarium will become sick if you neglect them.
Even hardy goldfish such as the comet goldfish will die if neglected.
The solution lies in equipping yourself with proper knowledge of goldfish care, networking with goldfish enthusiasts, and taking immediate action when you suspect sickness.