Because each step in the food chain tends to concentrate any substance that bioaccumulates. One example is vitamin A. Fish have lots of it in their livers, seal

Because each step in the food chain tends to concentrate any substance that bioaccumulates. One example is vitamin A. Fish have lots of it in their livers, seals eat fish, polar bears eat seals. Polar bears have so much vitamin A that eating the liver of a polar bear is almost certainly going to be fatal. So if your food waste contains heavy metals (as another example) and you feed that to the ant larva and in turn feed the ant larva to fish, you're likely to end up with fish that have high levels of heavy metals.