Are you using CO2 ? What are your light levels?
I upped my CO2 and ferts to get rid of algae. Search the archives for algae/fertilizer/Tom Barr or just wait, you'll get lots of response here. ;-)
Bob
If the tests are correct, NO2 killed your Oto and will kill all the others if not dealt with quickly. In my experience, Otos are *very* sensitive
Chuck H.