OMG, that self-poisoning could definitely play a role. Will do a much larger water change ASAP.
Also, is it possible that oxygen levels in the bottom of the tank are too low for them because there are bacteria sitting in the gravel.
Higher levels of the tank would be less affected explaining why ...

Please share your wisdom!
We have a tank that's been running for 2 months, fully cycled for 6 weeks (14.5 gallons). We have 5 healthy and very active Blackskirt Tetras.

We added 3 Julii Corys on Wednesday (they were sold as Julii, but look more like Trilineatus to me). They arrived active and ...