I agree with the others, it looks like air bubbles. Pre-soaking for two minutes with two drops of wetting agent added to the water will help eliminate air bubbles adhering to the film. If you're using a plastic tank, don't bang it too hard - you might crack it! BTDT!
What ever you do don't rap a plastic tank hard they tend to break or split, a gentle tap is OK.
Another APUG member gave me this tip, a while ago:
Get an old, unused mouse pad. One that you won't use for the computer anymore. Put it on the bottom of your sink or counter where you develop film and use that to bump your tanks on.
I always tap my tanks three times. Old habit. My first photography teacher in high school always told us "three times for good luck."
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2