Now that the DBT X CT Spawn has stablized with like 4 fry/ and everyone is sitting around under their bubblenest (exept the girls who are chasing the new female) I thought I should introduce you to the two most common problems I see with betta and how I treat them.
First off the best medicine is prevention and most of these diseases can be prevented and solved by cleaner water, this usually means more water changes and less feeding.
Fin Rot:
I commonly see this in betta coming out of the pet store or one that has been neglected for a period of time. For mild to medium cases I usually start doing 100% water changes everyday and adding a little freshwater Aquarium Salt. Severe cases usually will require I medication. Fin rot usually presents its self as a dark edge around the fins and will in a short amount of time manage to eat the edges of the fin in, till the point where it starts eating away the body... I have seen this before and its not lovely usually the fish will die as soon as the rot starts eating away at their body.

As you can see here is a photo of Splatter this is a photo of him healing and his fins are growing in clear (you can see this best on the bottom) and on his dorsal fins you can still see some of the dark edges of the fin rot. This is sometimes harder to spot on darker betta but its the same idea and almost all finrot looks the same.
Swim Bladder Disease and Bloating:Caused by overfeeding and bad water quality. This is real simple to diagnose the fish will start to have problems controlling their buoyancy. Bloating can cause swim bladder disease, and you will notice a fat tummy and lack of pooping. Swim bladder problems will cause a fish to float to the surface all the time or sink... mainly you will notice a lack of control in the fish.

In this fish here you can tell he is bloated the best course of action would be to raise the temprature and fast him for a few days. Also Epsom salt in 1tsp/gallon dosses helps. With swim bladder disease you can add epsom salt in slow upgrades to maybe 2 tsp/ gallon and see if that can help. Swim bladder disease can also lead to dropsy which is usually fatal.
Photos (c) to their owners.