Try this:
Mix 1 gram of Benzotriazole into a bottle containing 100ml of water. You have to shake the mixture mightlly because it does take a few minutes of such vigor to go into solution.
Then, start by adding about 10ml of this BZ solution per liter of working solution developer. That is not much and might not give much solace with reducing fog. For heavily fogged film/paper, I use about 50ml per liter of working solution developer. The slowing down of development with this highest percentage addition is substantial, but not profound. Of the cuff, I would say that you must increase development by about 33% to 50% with this '50ml' addition. I do promise that you will be pleased with the results.
My WORST film is Kodak Recording Film. I rate this old, heavily age-fogged stuff at about EI 16 and am pleasantly surprised with the lessoning of fog, even with the necessarily increased development needed to attain adequate contrast. Experiment and take accurate notes. Decide upon a particular developer (D-76 is fine) and stick with it so that results will become predictable. Different films might need different strengths of BZ added, but even with the max (ie, 50ml, or even MORE!) you can do ALL films even if, with lesser fog problems, that amount might be a bit wastful.
NOTA BENE: Yes, BZ can also be used with COLOR film and paper developers to aid in age-fog reduction. HOWEVER, beware of the slowing down of development that a little BZ can do with color chemicals. Start by adding only about one tenth of the BZ solution which I advised to add for traditional B&W. (There are too many different situations out there for me to offer more precision to such quantification.)
Finally, potassium bromide can ALSO be used instead of bensotriazole. Again, off the cuff, I would suggest using 10 times more (ie, 10g per 100ml water) as your stock restraining solution. This should then offer about the same effects as the BZ solution, so the ml additions to working solution developer will be about the same. - David Lyga