I am sure this is not a new issue. Like many folks starting out I mixed up a gallon of developer, stop and fixer for film work. In retrospect I realized that my film work starts and stops in spurts, so my film developer gets old long before I have developed the prescribed number of rolls or sheets. I have switch to Rodinal for my developer, one use and it is disposed of but what about the stop and fixer? How do you know when it is too old or better yet, is there a one time use product? I though about mixing portions of a liquid stop and fixer but how long will the partially used container last?