Elevated levels of nitrite. Cause is inadequate biofilter, so that the Nitrobacter bacteria which convert NO2-NO3 are not yet present in sufficient numbers. Dead Fish, excess food or overcrowding, can also raise nitrite levels.
Important to find and rectify cause same. Nitrite is more toxic at lower pH values, so in fresh water problem is likely to be more severe. Can bring on latent diseases if not tackled rapidly. Should not be a problem, in well run farms or aquaria.