A couple of customers are asking me how to sell fish. I hope these few points will help you!
1) Remember that you can only sell adult fish
2) Drag the adult fish you want to sell to the "Dollar Tank" ( Tank $ )

3)You can then click on the tag "Tank $" to check out all the fish that you are ready to start selling.
4) You will actually sell your fish only when you are in the Fish Shop. Click on the "Sell Fish" button and start selling!

When I am in the fish shop it says:
Fish still for sale: 1
Price:
Species:

Should the price and species be blank?

Thanks!

Yes, that information only displays immediately after you sell a fish.

my screen looks different than the one you have on your post. Is there an updated version I can buy?

That screen shot is from a handheld version. You're probably playing the Windows version, which looks a lot different!