I don't exhausted stop bath would cause the problem... that would just mean your fixer ended up more contaminated than is would/should. Using no stop bath what-so-ever won't do anything either other than maybe lower the chance of the fixer working properly I think.. never tried this, but I only use a water rinse as stop bath. Your plan of fault finding sounds good.(posted 8782 days ago)Tony has a good point.. how do you wash the negs? and have you tried re-washing them... is the muck on the negs or part of them? Actually I think we're asking more questions than answering :)