The marketing price for all the fish in the game starts at its most basic or 'fair' listing price. Once you put the fish in the Isolation tank, and click the "Species" button, it will show you the price that the game itself has pre-programmed as the appropriate asking price for that breed. So it should already be there, and should be the fair price. However, if you want to boost it's price by a few dollars, that is your choice.