As Ally just brought up temp.. I'll ask tank size.....
The favorite method here for revovery and treatment is heat..
For ich and faster recovery of other things. It is an oldy but goody.
And it is the most effective in the long run. Meds are hard because you have to know for sure what you have or you wil just end up giving the poor fish heavy metal poisoning without ever touching the pathogen.
And the disease is strong it will allow a med bath to take effect.
For such a known to be hardy fish; Bettas are actually surprisingly susceptible to some meds used easily on other fish. Coppers are okay. They were effective and safe (as a heavy metal CAN be) for bettas even before todays more controlled doses.
Disease control in modern bettas is not my strongest suit..... I never even kept one so inbred and disease susceptible until I got my modern pet betta for Christmas. My daughter is breeding for show though and getting insider tips to have her up to date on betta keeping in the millenium. And those I can pass on as well. My main experience is rare species care, breeding and preventative..
And heat is best
in your situation; because of the medicated soup that "backyard" betta breeders keep their crowded fish in and the the poor treatment on their way to retail locations..
There are also two new strains of medication resistant and betta-first ich and gram negative bacteria running like gestapo over some well known and healthy lines. Columnaris is getting to be seen more often than before in bettas (resistant #3?)
Go to the how-to section and use the heat method. Bettas are indeed a tropical fish and love heat. Your temps are nice low range for them.. Because they are adaptable and hearty doesn't mean the common method of keeping at "room" temps doesn't stress their systems.
Pet store Betta will often stress if there are no untaken hidey holes in a community tank, and males are often the target not the agressor because they are slow and have to expose theirselves at the top when they re-fill the labyrinth. Also most pet bettas are pretty much older adults or at least a year old for the size (Giant betta are a new strain and no one shelling out for them yet is in the commercial breeding business..) and the average pet betta only lives a mere 2-5 years.
Melafix is VERY bad for bettas...that is why Bettafix had to be invented.
Bettas are often in pretty bad shape when people buy them. They seem fine until you see what a healthy one of the same type and color looks like. Because a community tank can stress a Betta (they like space..don't really mind company..if
they have a corner to call their own) they need a pretty good long quarrantine and some feeding up before putting them in a community. A male and female may even fight when you are not looking. So many suspicious fluffs of healing slime over shredded areas will emerge around gill plate, eye, lip and torn fins. I've seen some on gourami and panicked to identify it too... ^_^
Many bacterial diseases have always hit bettas hard because if it gets through the gills it has the moist warm labyrinth cavity to make as home.
The Betta were always kept as tropical fish until the introduction of the first commercial veiltails. So had many years with great conditions to get their bullet proof reputation. As a kid we always kept them as tropicals. Until, like I mentioned, I recieved my betta in the unheated mini tank this Christmas.. I never had a sick one... And I keep all my fish now at 80
So..It is really hard to say what is wrong yet. It depends if they seem to get sick overnight or over the the course of the week..