We definitely have some rot here. I can see the necrosis in these pics, more so than I could in the other pic I saw. At this point, I would agree with Aus about trying Kanaplex/Furan-2 combo, since parts of the rot are creeping too close to his body for comfort.
Kanaplex can be pretty hard to find in-store and most often has to be ordered online. I recommend www.drsfostersmith.com
, as the shipping is fairly reasonable (you can combine products from different pet categories so if you want to get cat treats or something for Duff, you can
). API Furan-2 is much easier to find, at most Petsmarts. You can start with Furan-2 and see how it goes and if he doesn't show any progress with just that, get the Kanaplex.
In the meantime, Fungus Cure won't do anything. As Aus said, the condition it's meant to treat, columnaris, isn't even a fungus, it's a bacterial infection. To the best of my knowledge, Fungus Cure won't even treat the one true fungus that bettas do get, saprolegnia. As for Maracyn . . . best to wait for Furan-2. Both Maracyn meds are not very effective medications anymore, as they have been used so often that many bacteria have developed a resistance to it. In addition, it is not effective in waters with high pH. It can potentially become quite toxic if used after its expiration date or if a mixture is premade before use.