One more issue that has affected a couple of negatives: Dark circular areas in the sky, one per frame. These two scans are from adjacent frames, but there is one other frame that is affected on a different part of the roll. These spots are very difficult to see on the negatives but if I look very closely I do see areas that appear to have reduced density where these 'blobs' occur.
These seem too large to be air bells and I'm pretty careful to rap the tank on a hard surface after every agitation cycle. I also figured I would have more than one per frame if air bells were the cause and it seems too coincidental that they'd be in almost the exact same place in the two frames. I also thought maybe drying marks, but I don't see them when examining the negatives and I thought drying marks would look like lighter areas in scans. I'm washing my negatives using the Ilford method and doing the final rinse with distilled water and a few drops of Photoflo. I do have very hard water in my area, so perhaps I should mix my chemistry and perform all wash steps with distilled water.
Interestingly enough, the first few rolls of film I developed had no such spots and I used 3 drops of Photoflo in my 2-reel tank. For this roll, I decided to cut back to 2 drops since I was getting a lot of bubbling during the final rinse. I'd be surprised if using one less drop would cause this, but who knows. In any case, I re-washed the film just to be sure and there appears to be a bit of improvement (see 3rd scan), but the scans look a bit different despite me not modifying the raw scans at all.
I'm really committed to home development, but I'd be lying if I said I wasn't getting discouraged.
Here's a rescan of the second negative after re-washing: