Sudden weird deaths

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.

 

You said you only found 4 which could mean that one of the dwarf cories died unnoticed causing the ammonia to spike which killed al your endlers and your cobra. I would look for anything that might be decaying or any dead fish that you missed when you checked on them. If you can’t find anything then remove the decor and check the filter.

 

Thank you. I'll do that.

 

You said you only found 4 which could mean that one of the dwarf cories died unnoticed causing the ammonia to spike which killed al your endlers and your cobra. I would look for anything that might be decaying or any dead fish that you missed when you checked on them. If you can’t find anything then remove the decor and check the filter.

You seriously think 0.25pmm ammonia killed fish? No way.
I agree with looking for dead fish to prevent further decline but I’d think you added something nasty with your new fish.

Your nitrates are fairly high indicating to me your water change schedule is not adequate.

 

The ammonia could have been higher causing the nitrates to rise. I don’t think it would be a sickness or parasite but it could be, or a chemical.