We recommend the use of salt for external parasite and bacterial infections. It doesn't take much salt to harm plants so it's best used in a quarantine tank or one without plants. We recommend starting at 1 tablespoon per 3 gallons and working up to 1 tablespoon per gallon of water in severe cases. Remember salt doesn't evaporate and only leaves your system by changing water.
For detailed treatment instructions, read our article on aquarium salt.