I have a 20 gallon long, set up for over 2 years, lightly planted. I had 6 or 7 endler males and one cobra male that I've had for well over a year and 1/2. I added 6 dwarf cories two weeks ago today. I lost 1 early on but the rest have been fine. I did my normal 10 or 15% water change last night and this morning when I fed them, all was well. I just went to feed them tonight and all of my endlers and my cobra are dead. I found 4 of my cores which seem to be fine - the one I'm missing tends to hid - he's very tiny. When I checked my parameters, my ammonia was at 0.25, nitrites 0 and nitrates off the chart at 80ppm. I can't figure out why the guppies all died but the cores (at least so far) are ok. Would it have taken two weeks after adding the new fish for the cycle to crash? I did a water change and added Prime.