Betta fins suddenly shredded

Hmm.

You got me stumped Elaine. I can't answer the cyanobacteria toxin
question without some research. So, my first guess is...

1. cleaning the filter and doing 50% water changes could cause a mini
cycle. If your tests are old or if you're using dip stick tests you
might not be getting accurate readings. Perhaps there's a problem with
the test. (I'm totally reaching here and I don't think this so
probable, but ya never know)

2. Maybe they did just what you said... jumped... fought and jumped
back. It's TOTALLY not impossible. I lost a betta once. It jumped out
of a whole the size of a quarter. A QUARTER! They can maneuver through
impossible spaces. Plus, they both are showing signs of tattered fins.
Chances are if it were fin rot, one would show signs first then the
other.

Like you, I use clean water and TLC to heal shredded fins. I don't
indorse salt baths or Melafix. Others really like Melafix and find it
helpful when healing fin damage not caused by fin rot (bacterial). I
have heard of allergic reactions and possible damage that could be done
to the labyrinth organ though, so I don't bother with it.