Sorry to hear about your fish. I'm not familiar with Dats. But if I had a fish kept by itself, and it started showing signs of fin erosion and then a filmy white growth on it's forehead, after checking the water parameters (pH, ammonia, nitrite, nitrate), I also would do a water change. Another thing is check the water temperature. I believe both fungal and bacterial infections are mostly opportunistic attackers. There usually is an underlying cause for the sudden flare up. New fish added (either companion, or feeder), certain live foods, or water quality issues. Cronic fighting among tankmates causing open wounds are another opportunity for these pathogens to gain a foothold.
Before adding any medication I always make sure the water quality in the tank in question is within safe limits. That includes housing the fish in suitably sized aquariums. In other words, is the fish too big for the tank? I'm running into that particular problem with my female Vieja synspila. She's in a 75, but with companion fish and tank decor, she's looking a bit cramped. I'm not sure what I'm going to do.
Anyway, getting back to your problem. Like I said, I've no experience with Dats, not even sure what it is - Siamese Tiger fish? Or something like that? But most tank problems with disease are treated with the same basic steps. Check the environment, change some water, keep the sick fish a bit warmer, and if they're eating thats great. Try to identify the pathogen before dosing the tank. I've read fungal infections are rather rare in aquariums. Bacterial infections are more common. However, I'm no Vet or anything.
I noticed you mentioned adding salt? Are Dats brackish fish? If so, that may be another avenue to check.
I'm sorry I can't help more, but I hope I have given you some ideas to work with.
Bob