I have been losing a surprising number of cardinal tetras in my quarantine process. I started with 65 and have lost 18 so far, with a few more that are withering away. The fish had quarantined at the store for two weeks and were treated with metronidazole. There were no loses at the store and the fish were eating well at the store.
When I brought them home I did have an ammonia spike (I am assuming due to the dead fish), but I am looking for any other potential issues. The fish that die appear to just slowly wither away. They will lose their color, have tattered/disintegrated fins, then sink or float upside down until they are finally dead. I am not seeing any external sores or parasites, and I have not seen any aggression.
Timeline:
5/6: Fish added
5/11: Quarantine Medication Trio added
5/15: 6 dead cardinals
5/16: 7 dead cardinals, one dead otto, ammonia reading almost 1ppm, 50% water change
5/17: 5 dead cardinals, one dead otto, ammonia reading 0.5 ppm, 50% water change
The pH has dropped from 8.4 to about 8 since the fish were added, but the other parameters have been stable at:
pH: 8
Nitrates: <20ppm
Hardness: 300ppm
Nitrite: 0ppm
Ammonia: <0.5ppm
KH/Buffer: 300ppm
Water Temperature: 73 deg F
Any suggestions are welcome. It feels like I am missing something, but maybe this is just part of the process.