Betta Fish Diseases
Dropsy
Look for two signs: an abnormally big (bloated) belly and if you look at betta from the top, raised scales. Scales will look like an open pine cone.
Fungal infection
The first symptoms you will notice will be cloudy white patches on their head and body. Your Bettas will tend to be less active than the normally are. Some Bettas will stop eating and start losing their color. These parasitic colonies can quickly overwhelm your pet's immune system and inhibit it's ability to breath oxygen. Several aquarium treatments exist for this condition.
Tail or Fin Rot
As the first sign of tail rot or fin rot, Betta's fins start getting shorter or they start to separate. His tail and fins may also start to turn much darker around the edges.
Ick
It appears as little white dots that are sprinkled all over from eyes to tail. Some live food that you feed your Betta may be carrying ick. Ick is contagious and if not treated as soon as it is noticed, it could spread to other fish in the tank.
Velvet
The main indicator that your Betta has Velvet is a copper or rust colored dusting over the betta's body. Bettas that have Velvet will be lethargic, show clamped fins, and will have loss of appetite.