Fish will have some level of mercury, and we do test for mercury, histamines and polychlorinated biphenyl (PCB). Yet, not all fish were created equally. In a nutshell, the lower on the food chain, the younger and smaller the fish, the less mercury the fish will have. We use fish that meet the FDA’s list of those lower in mercury. There are many species of tuna, some of which have high levels of mercury; we use skipjack, a smaller and younger species that has lower levels.