Ran into my first puft, while working over a large contaminated air/water reserve. Since it wasn't hostile, seemed actually friendly, I basically ignored it. I was a little concerned when it flew up to the airlock and opened it but after a bit it flew back to the contaminated reserve and started converting slime.
Cool. Useful and a personality I could live with. A few minutes later I saved the game ... and the puft was never seen again. When I got back into the game, the puft was gone.
Haven't tried this again (should have thought to test by reloading an older save, but oh well).
Have a puft in the area, save game, leave game, load game, fish is gone.