If the fish die, you may as well shut the tank down, sterilize it properly, before starting again. If you want to save your fish, the treat them. Post a picture to get a diagnose.
There may be a way of treating without too much investment. If it is columnaris, lowering the temperature down to slow the bacteria may help additionally. It does not like salty conditions either, but you have a pleco and corydoras that do not tolerate salt, but they should be fine for a treatment like that for a while. I have personally used salt with my pleco and he was just fine. It is worth a try though, since it is an emergency. Do a large water change and if adding salt, make sure you melt the salt properly first, then add the dechlorinated salt with water very slowly. Max dosage 1tbsp per 5 gallons.
That's until you get a proper diagnose and treat with proper meds. Don't worry about the java moss for now.