I need help! Nems won’t last

I used to have an “infestation” of Chicago’s and Colorado’s and they were splitting like crazy. Now they are not growing - in fact some melted! Possible that I’m running too low in nutrients (NO3 < 10ppm, PO4 < 0.10ppm)? This is on my frag tank, now plumbed into the main tank. Possible that nems released toxins and nuking each other? I do run a little carbon (for Ozone). Any ideas?