The smaller the fish the less toxins

[Vince]

Fish can concentrate extremely high levels of chemical residues in their flesh and fat, as much as 9 million times that of the water in which they live.

https://www.peta.org/living/food/toxins-fish/

Stick to fish lower on the food chain (small species fish) to avoid contamination from mercury and other environmental toxins.

http://greenopedia.com/healthy-sustainable-fish/