Almost everyone that comes into my store seems to be looking for one kind of fish, and i do not know how to get it. Blue fins, orange tail, and looks long and skinny.
Any help would be greatly appreciated.
Thank you.

That would be a twin-fin fruitfish. You can check out the Fish Tycoon Guide Section for some breeding guides.

Some of the pictures on the poster and the people's thought bubbles are from other versions of the game, however, and that fishie is from the Palm and Pocket PC version. It looks different in the PC version,

That 'thought bubble' just means they are looking for fish, in general, not necessarily that particular fish.