Even if there is something wrong with the present filtration system, why didn't I get cleaner negatives when I used only distilled water?
Since you posted an example in an (there was a url link here which no longer exists) (that I missed previously), could you check the base side of that negative*? It's not very probable, but drying marks on negatives can have that shape, although they usually fade and don't show up as perfect lines.
* Against light, it reveals even the finest scratches.

