Interesting. Well…..not sure then what i had going on. Whatever happened, happened fast.
I tested when the fish started to die and they looked good to me:
Ammonia - 0 ppm, Nitrate - 0 ppm, Nitrite - 5-10 ppm, Water temp - 78, The tank has been up and running for six years with minimal issues.
The fish that died almost all had red bellies/flanks. They looked like internal bleeding/bruising. They seemed to be acting lethargic before dying. It seemed like the cories were breathing fast/having a hard time breathing. None of the fish had any external signs of problems. All I know, the fish were fine before putting the cories in the tank. I think the first Cory died within 6-8 hours. Then it was all downhill from there. Most of the cories died within 3 days. Then the other fish started to die.
Thank you for your help. I appreciate it.