I'm surprised the juvi's are dying too. I know some Neo lines are more tolerant than others. Not saying blues are tougher than yellows or anything like that, just some handle nose bleed levels and some, well, you're experiencing it. For example: Tom Barr had some amazing Bloody Marys years ago with tons of CO2 and 50%+ weekly WCs. They bred like crazy. Have also seen Bloody Mary's crash after a +/-30%, but diligent, water change
Once I went a little too heavy on CO2 and noticed my Neos going crazy and my drop checker was yellow. I shut CO2 off, and lowered my water level so my filter agitated the heck out of the surface and everything was normal in an hour or so. As ruthless as it sounds now I just run my CO2 til the checker is green and do big water changes and go Ivan Drago on them. It's interesting how some lines are tough as nails and some are delicate to changes. When I first got into Neos, my first attempts would die if I looked at them wrong and I would read/see how bulletproof they were for others. So frustrating. I'd like to think you could "toughen them up" over time, but not sure what to do other than what you are trying.
And jeez, dying at 15ppm sucks. Any chance there can be "bad batches" of CO2? Like exposure to copper or something?
92% of all statistics are made up on the spot.