Stress affects the immune system and weakens disease resistance. It therefore is – just as in humans – one of the main factors for fish disease outbreaks. Stress is caused by different factors. Maintenance mistakes (too many, insufficient or wrong measures taken), a fish stock that does not harmonize, adding too many fish, strongly varying temperatures,
overfeeding and correspondingly high organic water pollution along with a large number of
pathogens in the water are among the most common ones.