What are these colored speckles on my film?

Has anyone seen this before? My latest batch of C41 has these prismatic colored speckles on the emulsion side only. The base side is clean, no water spots or anything. I processed about a dozen rolls, but the speckles appear only on about half the rolls. They aren't black but appear multicolored in reflected light. This Ektar was not expired, and it seems to have been fairly random as to which rolls were affected. The speckles appear mostly transparent when viewed through the film, but they appear colored when looked at with reflected light (apologies for the poor picture, it was difficult to show the extent of it). They don't feel like they have any texture, and an attempted cleaning with some Pec-12 yielded no change/removal.

I'm using stainless reels in a water bath with manual agitation, using a preheat but no pre-wash/Fuji Negacolor Developer/Ultra Negative Bleach/Pro6 fixer/Rinse plus with added formalin for older films. Developer and bleach are replenished via Fuji's recommendations, fixer I track rolls and discard when it's getting used up (about 10-12 rolls). I use the standard processing times as outlined in Z131 and Fuji's documentation.

My first thought is emulsion-layer morsels stuck to the emulsion-side of your film, but this does not well explain them showing that peculier sight in reflected light.

Dirb9 said:

Rinse plus with added formalin for older films

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I think the cause is likely in this rinse, specifically the surfactant (so not necessarily the formalin). If you want to use a final rinse, try a solution of a little formalin in distilled water, and leave out the surfactant. If you add any surfactant, try only a little (a few drops to a full tank) photoflo. Or just use a pure distilled final bath instead of the final rinse.
Contemporary C41 films (let's say post 2000 or thereabouts) don't need the formalin bath.

PS: the good news is that these spots generally don't show up on prints. Maybe they do on scans; I don't scan much.

If it is a residue of surfactant, why then does it not show on the back of the filmstrip?

I don't know, but it's the best hypothesis I have so far. I've been getting these specks as well whenever I am a bit too liberal with the Adoflo, and they only appear on the emulsion side. Don't ask me how, I'm not a chemist...

I too suspect residue from the final rinse.

koraks said:

I think the cause is likely in this rinse, specifically the surfactant (so not necessarily the formalin).

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Formaldehyde is at room temperature gaseous at such will not form a solid.

I'd take a photo of the base side, but there's nothing to show, just the normal gloss with no white/water spots or the like. I mixed this batch of C41 chemistry about 5 weeks ago, I haven't had issues with keeping, but I used to use just Kodak Flexicolor and switched when it became unavailable, perhaps the Fuji keeps a little worse? I've never used distilled to mix the final rinse bath, since I filter my water to 5μm. I'll have pick some distilled and try rewashing and a freshly mixed final rinse, starting with say half strength (Fuji says 1:100, so I'll start with 1:200) and no formalin. I added the formalin since I do regularly process pre-2000s C41 film, and didn't see the need for separate chemicals for older and newer films, is it advisable to have a separate final rinse for each?
As for visibility, unfortunately it does show up faintly in scans mostly in the highlights, just enough to be distracting.