Tilapia
There has been a lot of controversy surrounding tilapia. First, understand that there are no varieties that have been genetically engineered for human consumption.
However, some sources of tilapia (particularly those from China, which provides over 70% of U.S. tilapia) routinely feed them animal feces (which will lead to Salmonella contamination) and use a number of chemicals (for example antibiotics, or sex hormones to produce mostly males which grow faster) to raise them.
But if you can find a good source, tilapia can add a lot of nutrition, but unfortunately they are not a great source of omega-3 fatty acids, which are the main reason diets high in fish are often recommended.