Below is a list of the most common goldfish diseases or ailments which could be the cause if you notice a fishes turn in behaviour:
Anchor Worm – If your goldfish is experiencing anchor worm, you will notice a ¼” (approximately 1cm) parasite (not so much one of goldfish diseases) that resembles a piece of fork tailed string hanging from the side body, head or mouth of your goldfish.
Dropsy – This is considered to be more of a condition, than one of the goldfish diseases. If your fish has Dropsy, its abdomen will become swollen, which makes their scales stick straight out.
Fin or Tail Rot – If you have feisty goldfish that enjoy attacking each other, you may have discovered that their fins have taken quite a beating. In this case then, not one of the goldfish diseases, however, if you don’t have other fish, or your usually friendly goldfish suddenly is showing signs of a badly frayed or torn fin or tail, this is usually a symptom of fin or tail rot.
Fungus – If your goldfish has fungus, you will know because different sized gray or white fuzzy patches will appear on random areas of the fish’s body. Not a cause of goldfish diseases, but usually a secondary infection.
Ich/Ick – This is considered to be the most common parasitic goldfish diseases also a blight of all fish species. It thrives among freshwater fish, and attacks those that have had recent infections,have weak health or low immune systems. One of the goldfish diseases theat is EXTREMELY contagious!
Skin Flukes – This is an ailment that can infect almost any fish, and is usually present in the gills.
Clamped Fins – This is usually caused due to poor water quality, but it can also occur if your goldfish is infected with parasites such as flukes.
Swim Bladder – This is a condition that affects the fish’s equilibrium. It can be caused for a number of reasons such as goldfish diseases that lead to the inflammation of internal organs, wounds from fighting other fish, poor water conditions and even constipation.