streaks in scans of black and white negs on flatbed

See the vertical streaking in the sky here. This was an overexposed Acros negative, developed in XTOL. It was scanned with an Epson v500. Is this a scanning thing, or a development thing? The negative is too dense for me to see a subtle variation like this with the naked eye. I don't have an enlarger to try printing it optically. I develop using a single-reel GAF cylindrical tank, the kind where you use the thermometer in the centre hole to swish the reel left and right for agitation. Someone suggested to me that these are caused by eddies in the developer from the swishing motion. Should I switch to a sealed tank that is inverted to agitate?

from what I can see this looks like a developing issue or less likely dirty glass.

switch to a sealed tank that is inverted to agitate.

I'm not familiar with that tank but why not try other negatives from the same roll of film and from other rolls to rule out the scanner. It appears that the streaks are pretty evenly spaced. I would think that eddies (if there is such a thing with your tank) would not be so regular. Also would a very dense negative yield such a tonal range? If it was a bald sky on a clear day I would think the sky would appear lighter in the positive. At least on my monitor the picture is not very contrasty. More information on the post processing would be helpful. Again test with other scans and perhaps someone familiar with that tank will chime in.

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It looks like a development issue to me. Turn the negative 90 degrees and see if the scan looks the same. If it does it's on the film. My guess is this is 120 film. For that I'd either use a Jobo (rotary motion) or the standard stainless steel tanks with spiral reels. You should get even development with either system. The SS tanks with standard agitation will probably give slightly sharper results due to edge effects with most developers. SS tanks are also cheaper and I find easier to load (though not all agree).

Thanks for the tips all. I did the 90 degree test and it is indeed on the negative. I need to look at my agitation technique and tank now.

Are you using a pre soak when you develop your films

Paul

@Paul-H, No, I don't presoak. I heard that that lengthens the development time, as it takes extra time for the developer to soak into the emulsion when it's saturated with water to start with. Does that help even things out?

Update: I got hold of a Patterson tank, used inversion for agitation, and the first roll through it looks good.