Over the past decades the aquaculture sector has seen a tremendous growth and intensification in order to meet the growing global demand for animal proteins. This intensification came with a cost as it resulted in an increase in disease incidences due to several factors. A common countermeasure for the increased disease incidence is the use of antibiotics. β-glucans are a waste product and can be used to improve immune functioning in aquaculture fish. Optimal use of β-glucans might reduce the need for antibiotic usage in aquaculture. I have used different approaches to investigate the mechanism through which β-glucan-supplementation modulates the immune system of carp. I describe long-lasting effects on specific immune cells and effects on the gut bacteria. By gaining insight in the mechanisms, we might optimize supplementation strategies further improving the health and welfare of aquaculture fish and reducing the need for antibiotics.