When a fish is being still in one spot, it definitely isn't floating or hovering. Poised? Suspended? "She made out a school of tiny fish ____ in a patch of filtered sunlight".
Why not hovering?
to float in the air without moving in any direction
to stay very close to a person or place
to stay near a specified point or level
It seems to capture both stillness but also the fine flutters and oscillations you see when you spot a group of minnows just hanging out. Plus, it brings to mind motion through air, which, when talking about fish, is an imaginative touch.
or Wavering, maybe with a modifier? (barely wavering? hardly wavering?)
to sway to and fro, to tremble
blinking, quivering, winking, glimmering
I think floating immobile may describe the scene:
- Buoyed on or suspended in or as if in a fluid. (AHD)
- Periodically, fish rose next to the bank, but they were not taking anything ... that move on the water instead of floating immobile before springing into flight. From: A Guide’s Guide to Fly-Fishing Mistakes: “Presenting the Fly”
11) to come to a stop, usually temporarily; cease from operation for a time.
14) to be suspended, as in a liquid, gas, etc.