There's no "general" medicine. The most "general" you could get would probably be something like aquarium salt it really depends on what kind of sicknesses you are dealing with
When you say you use salt, do you just use it to prevent sickness and for sickness? I'd like to know the amount and when to use it to prevent sickness.
Yep, I use it for both. I usually put in 1/2 teaspoon per gallon for disease prevention, and 1 teaspoon per gallon when treating an ill betta. I add in the drop of aquarisol (per gallon) and salt every 100% water change.