Quote Originally Posted by keith_r
i know uf does not agree...
but i read elsewhere that while many fish are ok with high nitrAtes, catfish can have problems.. especially when fingerlings.. i'll try to search for it today, but it's gonna be a busy one at work
Oh, I wasn't commenting on if nitrate toxicity exists in this particular fish species... I was just trying to keep it simple because in aquaponic systems nitrates are relatively low (under 100 ppm, often under 40 ppm or some people have near 0 ppm). Save the poor guy a test and confusing anyone else that might be trying to help. I am now in a more "hands off" mode. I'm passing the torch to you keith!!!

Your test should read the TAN (total ammoniacal nitrogen), which is NH3/NH4 combined. NH4 speciation is associated with pH (I may be stating the obvious for some).

NH3 - unionized ammonia or just ammonia (toxic)
NH4 - ammonium (non-toxic)
NO2 - nitrite (toxic)
NO3 - nitrate (non-toxic or weakly toxic depending on species and stage in life cycle)

DISCLAIMER: And yes, I know speciation occurs for the other chemicals too and more speciation than listed also occurs, but this is the bulk of the relevant and significant interactions with the nitrogenous compounds.