I've been doing some Van Dyke brown recently, which is similar in many respects to Kallitype. Bleach back in the fixer can be a problem with either process. Be sure to use a dilute fixer. I use a 5 percent sodium thiosulfate solution, but many recommend 3 percent. Do not use ammonium thiosulfate. For VDB, where the silver sensitizer washes out in the developer, fixing is limited to one minute in the diluted fixer. Read up on the process a bit to see what is recommended for what you are using. I have been toning in GP-1 before fixing for my latest batch. It works decently well, with only a slight change in image tone toward pink. Unlike untoned VDB, there is practically no change in color when the image is fixed. My guess is that extended fixing would still bleach the image.