Keith:(posted 8475 days ago)An acid fixer will stop the chemical reactions of the developing agents the same way an acid stop bath does. As a matter of fact, in an old darkroom instruction book that I have, acid fixers are referred to as "stop-fixers". If some sort of stop bath isn't used, however, problems can arise. Excessive developer carry-over can cause stains, and changes in the pH level of the fixer can reduce the fixer's effectiveness.
If an acid stop bath isn't used, it is still wise to use a water rinse between developer and fixer. It would also be wise to moniter the pH of the fixer as well as periodically check film clearing times. For large-volume work, it might be necessary to replenish the fixer periodically.