In aquaculture, fishes and invertebrates are exposed to a number of substances in their diets that are not normally found in their natural habitat. As marine reserves are limited, sustainable plant-based protein sources for aquafeeds are being explored. Phytosterols and mycosterols are commonly found in the feed source. While several groups of invertebrates require dietary (phyto)sterols for their development, the picture is less clear for fish. Both growth promotion and disruption of sex hormone homeostasis have been reported with conflicting results within a given species. It is reasonable to assume that some of these conflicting results are due to the fact that sufficiently graded dietary levels of phytosterols have rarely been applied. The same may be true for zearalenone, the most abundant mycosterol, for which only adverse effects on life history traits have been reported. Next, several studies on mycosterol mitigation are reviewed. Finally, we emphasize the need to include epigenetics in the understanding of toxicity and mitigation pathways.