That was a good list. Thanks. I didn't ask about water temp, or how it's measured (glass thermometer, fancy probe, or some stick-on-the-glass tape catastrophe) and you didn't say. That's a pretty crucial point of interest. If it's not heated and the air/water temp drops as significantly as the ambient room temp each night, that's a problem.
If the animal basks a lot (not always normal for this species) it may indeed have something going on beyond the leg affliction. Provide another basking site that is warmer yet (mid nineties) but don't remove or change the one that's working now, for the sake of giving options.
I've never kept this species as low as pH 5.0, but, that's low for many species of "Amazonian" fish, so, it might be low for your sympatric turtle. From what I've seen, fungus-looking issues can also be caused by pH that is too low. That problem is, however, normally with regards to high pH species rather than blackwater species. Perhaps, keep it at a comfy six or six and a half for now? Recall that acidifying treatments for fungus/bacteria used for aquarium fish (such as Aquarium Pharmaceutical's MelaFix) can cause ammonia spikes in the tank. High levels of nitrogenous waste can also be a cause of excessive basking (staying out of invisibly-dirty water). You want to find a way to take those measurements. I've known a person to use pH Down without issue in multiple species tanks, but I don't feel any degree of comfort towards the product. It is an extremely strong solution. Although I'm not a vitalist, I like to acidify water by using dry oak and hickory leaves - a very natural solution to a high pH. You can get them almost anywhere in your state (particularly this time of year), they are easily replaced as the tanins leach out, and they provide an environmental accent. I will include, they'll color the water to a tone more comforable for your turtle - a slight tea hue. Most Amazonian cichlid enthusiasts have come to embrace this hue for the dual benefit it serves their captives (correct pH and sight security, both of which are necessary for your turtle).
Be careful with the betadine, especially if it's not diluted. That is usually used on the shell rather than the soft parts with regards to turtles, and it can have a very abrasive effect on tissue.
Similarly, if it basks excessively, lose the UVB 5.0. This is not normally a strong ray species, at least not at duration, so that thing could wind up burning the animal. I know it sounds contradictory to take away the UV source on a potentially sick animal, but, burn it and those legs will have a whole new problem.
Fish will prove a fine diet, but they do carry parasites, and your animal, as a likely fresh import, may already have an excess of. Periodic treatment for this scenario has proven helpful for some with mata mata.
When a problem like this creeps up, start changing things. Don't be afraid to. You are bound to make mistakes. Everyone makes some with their captives almost (always?) every day and the only ones to be hit hard by this (keepers and animals alike) are those whose keeping regimes are not switched up with hopes for achieving a better tomorrow.