I've put a drop of water from the bed of my walleye larvae tank under the microscope at 400X and i've seen these rod-like bacterias. By its shaking mouvements, do you think this is Flexibacter columnaris (see the video : http://youtu.be/SYH4wZCw8wM) ? This is not appear to be "flexible", so I guess it's not. I've no sign of disease on fish for the moment. The bacterias seems to feed on food residue. (Freshwater: Temperature = 18°C, [O2] = 8mg/L.)