you had a co2 tank dump? Did you have a dual-stage regulator?
Nope, cheapo single stage. Yes, I shoulda known better. This is the only reg I've ever used on planted tanks. In the past, when a CO2 tank dumped, it would spike the CO2 but not enough to actually do any harm. I had assumed that this very large aquarium with tons of surface area and a fair amount of surface agitation would be far more able to handle a CO2 dump without a dangerous CO2 spike but clearly I was wrong.