What is this bubble in my res bucket?

Rdwc, 68-72F, 400ppm flora series, no light leaks. Ran h2o2 and had some of this stuff, switched to bleach at 2ppm and still have this stuff. Looked under microscope and it doesn’t appear to be algae. Plants and roots are doing fine.

I can't help but wonder what you did see under the microscope?

Here’s what it looks like on a slide at 250x. Comparison to a fan leaf at the same magnification.
Changing out the water today and running bleach through it to clean.