Looking at your photo it seems like the film base+fog is quite high. I've seen this with noticeably out of date film, so my question is: How old is your film? I have seen age fogged film that that looked like yours, I've also seen age fogged film was good on the edges and fogged in the center. I would suggest refixing the film in fresh fixer, don't skimp on the fix time, 10 minutes should clear any left over silver. By doing this you will eliminate the fixer as the possible problem and you can look elsewhere for the cause.