Alexander Gould was the voice of Nemo in “Finding Nemo”. But if you ask him to do the voice, you’ll be disappointed. He says, quote, “I have to explain that I was, you know, 9 years old and my voice has changed a little bit since then.”
Gould did Nemo’s voice until he was about 14, then he lost the gig because his voice had changed too much. Quote, “I wasn’t able to capture the essence of Nemo anymore. That happens when you’re a child actor.”
A younger actor voiced Nemo in the 2016 sequel, “Finding Dory”, but Gould had a cameo as a delivery truck driver.