They were alive this morning, but come nightfall all were dead.  The ammonia alert badge hasn’t said a peep, and is still relatively new and shouldn’t be having any issues.  Granted, it is testing for free ammonia, whereas my liquid test is testing total ammonia.
I test the water, and once again I’m confronted with atrocious readings that could more than explain the problem.  pH 8.0, Ammonia 0.5 ppm, Nitrite 5 ppm, Nitrates 20 ppm.  But I just did a large scale water change (roughly 70%), and the fish had gone through that unscathed.  So is this a reading of the “aftermath”, or is there something more sinister at play?
This time, I happen to have both the water I removed from the tank 24 hours ago, as well as some leftover water from the new water I was trickling in.  Both are tested….
The old water – ph 8.0, Ammonia 2 ppm, Nitrite  5 ppm (so it’s quite possibly off the scale in both tests), Nitrates 10 ppm?  I don’t know how the older water had lower nitrates than the tank after water change, unless that is in fact massive rapid conversion into Nitrate in the main tank from all the die off.  Just doesn’t seem right though.
Tested the makeup water, 8.0, no ammonia, nitrite or nitrate.  The RO/DI water is ph of 7.0, otherwise the same.
The most interesting part – Spawn #21, which was a weak run from the get go, still has about 4-6 fish hanging on.  Tested their water?  Nitrites off the scale once again.
My working theory is this – I may have dropped the salinity during the water change, and as a result, removed the “protection” afforded by higher salinities against nitrite poisoning.  That’s the best guess I have at the moment.