I am not a vet, and have no medical degree in diagnosing Bettas. I am simply an enthusiast with over 20 years experience. The information here comes from my own experience as well as reading I've completed, but I make no promise or guarantee about any of the info or cures below.
If you are not careful, your Betta can catch a number of illnesses. You will usually be required to separate your Betta if it is sick. This will allow you to safeguard your other fish in case the illness can spread. be careful to not have an abrupt replace the water by utilizing water that has been left out and at a similar temperature for at least 6 hours.
Make sure not to spread any disease by washing your hands with a purifying soap after touching a sick Betta fish.
Diagnosing your Betta's illness is crucial to giving him the proper treatment. The post Betta Fish Diseases and Cures discusses remedies when you've determined a proper diagnosis of the illness. Furthermore, it discusses methods to prevent the majority of problems.
A able-bodied Betta should be vigorously swimming in his tank, while an ill one generally hangs around in the corner or possibly near the bottom and may just come to the surface for air.
Symptom: Clamped Fins
Make sure to check his fins. If they are close to the Betta's body, then your fish may have caught clamped fins, which usually comes from poor water quality. Begin changing half of the water every day til the quality is better and his fins are healthy.
Betta Illnesses - Ich
Ich is a dangerous and spreadable parasite. If you notice small white dots on your Betta's body and fins, he could have Ich and require treatment immediately. If you see your Betta rubbing against the sides of the tank, chances are he has Ich and is trying to wipe it off.
Ich can happen because of inconsistent water temperatures or other stressful scenarios for your Betta. Just like in humans, Betta fish that are stressed tend to have many more sicknesses.
Illness: Fungus Contamination
Large dots of grey or white is most likely a fungus contamination. These growths will be bigger than with Ich. If it remains untreated, it could start to look like a fuzzy white growth.
Disease: Fin Rot
Fin or Tail Rot can be determined by looking at his fins or tail. If they appear to be shredded, are getting shorter, or have holes in them, then he probably has contracted Fin or Tail rot. This is not an uncommon disease a lot of Betta's get and must be cured as quickly as possible. It is a bacterial infection that can easily spread to the rest of his body. In really bad cases you can see bleeding fins.
Your Betta fish can get constipated if his food is not changed from time to time (Betta's are best when eating a variety of food - see Providing Food for Bettas for more instruction). If you notice a swollen abdomen, but besides that your Betta is acting like normal, then chances are he is constipated.
Velvet can be seen by the tell-tale gold and yellow powder like dust that will appear on his body. It may be difficult to notice, so if you're not certain, shine a light on his body to see.
Dropsy is easily spotted with the large belly and raised scales on your Betta. The scales will appear to look like a wide open pine cone if looking from above.
If you notice your Betta fish's eye popping out, if it's cloudy or has a bubble on top of it, then he has Popeye. It's easy to diagnose and very simple to cure.
Betta Fish Illnesses - Tuburculoses
Tuberculoses is the only illness that a Betta can pass to a human being, so make sure you don't have any wounds and are not already sick (with a poor immune system) prior to submerging your hand in the water, or better yet wear clean gloves. Do not worry too much - the worst it can do to you is transmit to you a skin infection.
If you notice swelling or scale protrusion, weight loss, skin defects and/or discoloration, scales falling off, loss of appetite, lethargy and clamped fins then he may have Tuberculosis.