Aquarium salt is always a must, Indian Almond Leaves also, make sure you have a small hospital bowl or tank (1-2 gallons) and a heater that will fit that size (10watts). Garlic is a good natural remedy for parasites, blanched de-shelled peas for constipation. I would always have frozen food handy because its slightly more nutritious than pellets and will boost your fish's immune system. Epsom salts help for different things.
These are all natural remedies.. I prefer to stay away from medications if at all possible but I'm sure someone will chime in with good medicines to have on hand.