Help! I have a weird issue with black spots showing up on my Efke negatives and could use your advice.

About two months ago I started a thread about black spots on Efke 25ORT. At the time, I thought it had something to do with an emulsion flaw, and moved on. This month I have had the same issue with Efke PL100, and am convinced that I am doing something wrong, but can't figure out what. Other films I have used recently (FP4+, Delta, Lucky 100) have not shown this pattern. I could use some advice problem solving.

The effect (see images below) is thousands of tiny black spots on the film. Usually these spots tend to show up medium to light grey areas, such as skin. I have noticed that black backgrounds do seem to produce spots. Also, not all the negatives I develop show these flaws, maybe half of them do.

I have used multiple developers (Xtol, D76, Pyrocat), and have seen the same effect with all, so it is clearly not in my developer. Perhaps my tap water?

Could this have something to do with my developing tank? I am using a Nikor 4x5 tank. The tank is, as best I can tell, clean, and I carefully wash and dry it between use. Maybe agitation? Again, not all of the negatives show these marks, but roughly half of the images per batch do.

Advice is appreciated!