When you are still trying to save coins, try selling your fish when the market price is better than the average. This is indicated when the arrow near the fish points up and turns blue. There is a man at the upper left side that always has a fish selling mission. Deliver the desired number and type of fish to him and he will pay twice the market value. Always try to complete his requests. Sell all the fish that he does not want and save the ones he needs until you have enough to sell them to him. You can even wait for the market value to be above the average so that you get even more money from him.