I have just opened a box where I have some of my b/w things and I found a container with fixer. When I opened it the fixer was colourless but the sides of the tanks are covered the some dark grey/black stuff...how can I clean this container?
Morten, since scrubbing is not a choice, you're left with chemical cleaning. There are formulae out there for tray cleaners and stain removers, but the chemicals involved are quite nasty. To be honest, I don't know if that stain is a problem or not. If in doubt, change the container.
I had what I think is the same black/grey stuff on the inside of my fixer slot of my NOVA Quad Slot Processor. I found that bleach cleaner that is used for household cleaning mixed with water worked. Just be prepared to leave it in the tank for several days, then rub the sides with an old toothbrush.
I am sure there are proper cleaners that will do the job better or quicker but household bleach cleaner is cheap and easily obtained and it worked without damaging the plastic sides of the slot processor.