what can a PSB do? does it help the water clean or nourishes fish?

Answer 1

You may not get too many answers on this, because I had to look this up. You're seeing this on the ingredient list of a fish food I'm assuming.

Azoo foods seem to be the ones that list this, and according to their website, PSB stands for PhotoSynthetic Bacteria: http://www.marinedepot.com/ps_AquariumPage.aspx?PageAlias=fish_food_azoo which they say are nutritious.

They don't specify which bacteria are used, but some of the photosynthetic bacteria are pests (cyanobacteria).