I have had my tank some months and all the levels are fine (NO2, NO3, Ammonia in particular) tank size 12 x 18 x 18.
I have/had 3 foxes - all healthy, 4 danios - 3 died in last 2 days, 2 platys - 1 died in last 2 days, 4 diamond tetras - introduced 2 days ago - all healthy, 5 neon tetras -...