Freshwater items have a different nutritional value than saltwater items. Much higher in fat. The effects are long term, you won't notice over night, but it will eventually lead to liver disease, possible blockage in the digestive system, and then premature death.
I can eat potato chips all day long. Is it going to kill me in a month? A year? 10 years? Eventually the cholesterol is probably going to cause heart problems, etc. You get the idea.