I'm not a copyright lawyer, so I can't give legal advice but really it you don't need to be a copyright lawyer to know that that characters such as Nemo are copyrighted, usually by the companies making the movies. And if something is copyrighted, you need permission from the copyright holder to copy it. If you started to sell the paintings based on copyrighted characters, you’re stepping into potentially dangerous ground. Rather create your own characters and sell these. Not only do you then not run the risk of violating copyright, but it's a more original too. Sure it'll take some work initially and a few versions, but it's worth it in the long run.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.