You can almost always sell fish for more than the initial price, especially when the selling tank is full or nearly full. I have found that trying to sell only a few fish at a time will result in sluggish sales. If you wait until you have a lot of fish that you want to sell, more customers will enter the store and stay there, and sales will be made more quickly. Only trial and error will tell you what the "best" price should be for each species of fish, since it's a little different every time you play the game.
Each area of research (food, fish environment, and advertising) is completely researched when you reach level 3.
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Barbara
Unicorn
Last Day of Work