i'll be in the minority here saying that you are a good fish keeper. Guys on this website are gonna flame you unless you have him in a 5000 gallon or bigger. 400 gallons is plenty for that nurse. When some guys see that photo they are going to have a pavlovian response and tell you need thousands of gallons. Its like when I put my black tip in a 560, I got flamed, but he was in a 300 for a year before that, so I basically doubled his swimming capacity.
Would he be happier in a 10000 gallon tank? Yeah, but I don't have $50,000 to spend on a tank like that. He is far happier in my 560 than he was in that 300, thats all i know.