Hi DeeKay,
My guess is that your pictus pigged out on the brine shrimp. That would explain the fat belly, and loss of appetite for other foods. Are there any other symptoms suggesting that the fish is sick? Some fish will eat until they seem ready to explode...lol. Oscars come to mind...and are probably the best example of overeating. Keep an eye on it...but don't worry too much. You probably just found it's favorite food.