Dimitar, I would suggest you use before the b&w develator a prebath - sodium carbonate 20 g / l. Time = 3 min. at room temperature.
Sodium carbonate or soda wash in our country is store to chemicals.
After prebath wash the film about 3 to 5 min, for not increase the pH of the b&w developer.
I hope that this will reduce the fog, that whites are cleaner.
Another option is to use b&w developer (Rodina) concentrated (1+ 15) for 8-10 min at room temperature.
George