Was the Stanford Prison Experiment ethical?

After reading up further on the Stanford Prison Experiment I don't think it was ethical. Zimbardo became an active participant in the experiment. There didn't seem to be enough controls in the experiment, beyond the structural ones. The guards received little guidance and few limitations on their behavior. Did it resemble real conditions as closely as it should/could have? Also, I'm not convinced there was full disclosure as to the treatment you may receive if you were selected to be a prisoner. Plus, all were compensated for their involvement. What would the pool of applicants been had they not?

It was surprising to read that each side slipped so deeply in to their roles so quickly. And with the number of visitors to the "prison" - including parents and other university staff no one thought Zimbardo had lost perspective and the experiment was out of control until 6 days in.