I try to sell the tilapia alive
....I don't eat much fish.
The fluid is likely as Bill said, from the gallbladder or another organ being accidentally cut open and tainting the meat.
Tilapia are well known for their flesh NOT taking on the off flavors from any of their foods, like most animals do....Letting them purge (live) in clear water with no food source of any kind is always the easiest way to clear the intestines.
