I have looked all over the web and i cannot find one cheat that has worked.. this is just a way to make easy cash.. you need to research your environment to breed rarer fish and make money.. right? WRONG! when you breed a rare fish you will see his health going down
if he survives long enough sell him! they will buy him! just make sure you do it quickly and that you arent trying to sell a dead fish.. :S

hope this helped...