Taken from this report: How Tilapia is a More Unhealthy Food Than Bacon.
They've Had a Sex Change
Virtually all tilapia sold in American supermarkets has undergone a sex change–the result of being fed methyltestosterone during the early, sexless stage of life. Tilapia pumped full of hormones grow bigger quicker than their natural bros, because they don’t expend energy developing reproductive organs and require less food. Seafood experts consider the effects of methyltestosterone in fish to be insignificant to our health. However, there’s research to suggest the drug can be highly toxic to the liver. In fact, methyltestosterone has been taken off the market in Germany due to its high potential for liver toxicity.