I don't know if the "freshness" or lack thereof mentioned by
@herfsty could or should be interpreted as an issue related to sharpness, resolution, resolving power or something else slightly more specific and objective. Perhaps he can chime in and make it more concrete.
As I mentioned before, I've let my film sit in a "stop bath" (running water) for up to 5 minutes, then fixed it. I still end up with tea colored fixer afterwords.
The noticeable difference is in the highlights, it is hard to describe, but the image doesn't come out as "bright," if that makes any sense. Less contrast, less punch. The sky on a clear day will be an extra shade of grey depending if fixed with used fixer. That's the only way i know how to describe it. I was working on a big project so just kept using fresh fixer every time I developed. I can begin experimenting more now.
Someone a while back mentioned that I should use distilled water throughout the whole process. I've done that and it's making a difference. I live in southern california. The water is filthy. So far I've only been able to experiment with 35mm, have yet to try 120.