Nova sells a special cleaner called "Tarbuster" which works well. If you get it then use the gloves and facemask that comes with it. Household bleach diluted at about 10:1 works as well. You just need to be patient with it. Leave it in the slots for several days and each day try the bottle brush. You'll soom know if the crud is lifting. If it isn't then give it another 24 hrs and eventually decrease the dilution.

I had very bad and stubborn fixer stains but eventually they lifted with no harm to the processor