the tiny
little bugs would float off the plant and then I noticed the fish eating
them.

That's a problem with my fish, they're pretty stupid and refuse to eat
bugs on the surface of the water. I had an aphid problem and would
wash them off the leaves, but the fish would ignore them. In a
previous pond, washed off aphids were considered a treat and hunted by
my fish! Not these! So I doubt they'd go after the mites but from
what I understand a mite won't survive a fall into the water anyway.

I love duckweed, but my fish WILL eat that.