Fish pathogens - bacteria, fungus and parasites are present in all ponds at all times.The fish do not become unwell because their immune system fights of the disease. When fish become stressed their immune system does not work as well and so they can easily succumb to disease.The major stress inducers include poor water quality, fluctuating water temperatures such as in spring and autumn and when new fish are introduced. When to use General Tonic. Monthly to keep background levels of pathogens down to a minimum and prevent fish disease. When introducing new fish In the spring to help fish recover from a hard winter. In autumn to prepare fish for the winter period.Treats 2250 litres