I apologise for this essay, lol. Its just kind of hard to say which disease is what without a microscope, really - so here's info that might help.
There's a couple of different diseases that cause body/fin rot. The main ones are:
This is a true fungal disease, as opposed to fuzzy bacterial diseases which are called 'fungus' due to the resemblance.
Saprolegnia (let's call it 'Sapro') is an opportunistic fungus that tends to attack fish which are already sick. It likes to feed on necrotic flesh - like the ulcers and dead fin tissue caused by columnaris or aeromonas, which we'll look at in a moment.
Sapro loves sick fish and poor water quality. It can kill fish - eventually. But it's a slow death compared to the bacterial 'fungus' disorders.
One way it manifests is in the 'fuzzies' - little white fuzzy tips on disintegrating fins which come and go, sometimes clearing up for weeks before returning. It can also infect body and fins.
Sapro is that white mould that grows on uneaten food in the bottom of the tank. It CAN go from there to attack fish if the fish are not healthy, which is just another good reason not to overfeed and to clean tanks regularly.
Sapro is usually less grey than bacterial infections. It can be hard to tell from columnaris, but as columnaris is the more deadly disease and is often the reason sapro is there in the first place, IMO it is best to treat for bacteria first and deal with the real fungus later.
It is best treated with Malchite Green.
Columnaris (aka 'fungus' 'cotton mouth' 'fin rot') is an aerobic, gram-negative bacteria which has a broad range of symptoms including ulcers, mouth rot, 'cotton mouth', fin rot, 'saddle back fungus', fuzzy patches, sores, popeye and cloudy eye.
Columnaris -loves- clean water because it is aerobic, meaning it need oxygen to survive. It isn't dirty water that encourages columnaris - this disease is most often caused by poor Redox/mineralisation, uneven temperatures/ temp spikes, overcrowding and stress.
It is best treated with proper gram-negative antibiotics (NOT the -fix medications, which are only antiseptics) like Kanamycin.
Columnaris doesn't like salt much, and if caught early can respond well to Aq salt treatment. But as it moves so fast, it's hard to tell 'early' from 'impending death' at times..
** There's a lot more to it than this, and it's really worth reading up on it as Columnaris is one of the most common aquarium killer-diseases.
This is another gram-negative bacteria, which likes ANaerobic conditions - oxygen-poor, dirty fish tanks without aeration and poor filtration are paradise for this disease. Ie, the majority of 1/2 -1 gallon bowls out there...
Aeromonas causes popeye, bloating, bloody spots and open sores, swim bladder disorders, intestinal problems, rapid necrosis and septicemia - it is also a major cause of dropsy.
It is best treated as for Columnaris - with kanamycin, only add to that neomycin or nitrofurazone.
** It's interesting that aeromonas loves low ph -- but doesn't like IAL, which is probably why wild bettas thrive in oxygen-poor water full of lots of organic debris without getting sick.
Here's some articles that really worth reading: