Probably a fixing problem. You should fix for at least twice the clearing-time. With Delta 100 (and T-Max), some people (including myself) prefer three times.(posted 9729 days ago)Dunk some film in the fixer. Time how long it takes for the film to go through a milky stage to totally clear, with no trace of whiteness. If this is 4 minutes, then it needs 12 minutes in total.
Of course, the fixer should be roughly the same temperature as the developer. If it takes 7 minutes to clear the fog (so it needs 21 minutes total), there's something wrong with the fixer.